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National Collaborating Centre for Acute Care (UK). Head Injury: Triage, Assessment, Investigation and Early Management of Head Injury in Infants, Children and Adults. London: National Collaborating Centre for Acute Care (UK); 2007 Sep. (NICE Clinical Guidelines, No. 56.)

  • This publication is provided for historical reference only and the information may be out of date.

This publication is provided for historical reference only and the information may be out of date.

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Head Injury: Triage, Assessment, Investigation and Early Management of Head Injury in Infants, Children and Adults.

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Absolute risk

Measures the probability of an event or outcome occurring (for example, an adverse reaction to the drug being tested) in the group of people under study. Studies that compare two or more groups of patients may report results in terms of the Absolute Risk Reduction.

Absolute Risk Reduction

The ARR is the difference in the risk of an event occurring between two groups of patients in a study – for example if 6% of patients die after receiving a new experimental drug and 10% of patients die after having the old drug treatment then the ARR is 10% – 6% = 4%. Thus by using the new drug instead of the old drug 4% of patients can be prevented from dying. Here the ARR measures the risk reduction associated with a new treatment. See also Absolute risk.

Acute sector

Hospital-based health services which are provided on an in-patient, day case or out-patient basis.

Advanced Paediatric Life Support (APLS) course

A course for healthcare professionals run by the Advanced Life Support Group which teaches a practical systematic approach to the management of acutely ill or injured babies and children. (See

Advanced Trauma Life Support (ATLS) course

A course with the aim to teach a simple systematic approach to the management of trauma patients through interactive tutorials, skills teaching and simulated patient management scenarios. (see

Algorithm (in guidelines)

A flow chart of the clinical decision pathway described in the guideline, where decision points are represented with boxes, linked with arrows.

Allocation concealment

The process used to prevent advance knowledge of group assignment in a randomised controlled trial (RCT). The allocation process should be impervious to any influence by the individual making the allocation, by being administered by someone who is not responsible for recruiting participants.


Partial or total loss of memory, usually resulting from shock, psychological disturbance, brain injury, or illness.


The extent to which the results of a study or review can be applied to the target population for a clinical guideline.

Appraisal of evidence

Formal assessment of the quality of research evidence and its relevance to the clinical question or guideline under consideration, according to predetermined criteria.


See Absolute Risk Reduction.

Basal skull fracture

A fracture involving the base of the cranium.

Battle's sign

Bruising which sometimes occurs behind the ear in cases of fracture of the base of the skull (basal skull fracture).

Best available evidence

The strongest research evidence available to support a particular guideline recommendation.


Influences on a study that can lead to invalid conclusions about a treatment or intervention. Bias in research can make a treatment look better or worse than it really is. Bias can even make it look as if the treatment works when it actually doesn’t. Bias can occur by chance or as a result of systematic errors in the design and execution of a study. Bias can occur at different stages in the research process, for example, in the collection, analysis, interpretation, publication or review of research data. For examples see Selection bias, Performance bias, Information bias, Confounding, Publication bias.

Blinding or masking

The practice of keeping the investigators or subjects of a study ignorant of the group to which a subject has been assigned. For example, a clinical trial in which the participating patients or their doctors are unaware of whether they (the patients) are taking the experimental drug or a placebo (dummy treatment). The purpose of ‘blinding’ or ‘masking’ is to protect against bias. See also Double blind study, Single blind study, Triple blind study.


Cervical spine or bony part of the neck

Case-control study

A study that starts with the identification of a group of individuals sharing the same characteristics (for example, people with a particular disease) and a suitable comparison (control) group (for example, people without the disease). All subjects are then assessed with respect to things that happened to them in the past, for example, things that might be related to getting the disease under investigation. Such studies are also called retrospective as they look back in time from the outcome to the possible causes.

Case report (or case study)

Detailed report on one patient (or case), usually covering the course of that person’s disease and their response to treatment.

Case series

Description of several cases of a given disease, usually covering the course of the disease and the response to treatment. There is no comparison (control) group of patients.

Causal relationship

Describes the relationship between two variables whenever it can be established that one causes the other. For example there is a causal relationship between a treatment and a disease if it can be shown that the treatment changes the course or outcome of the disease. Usually randomised controlled trials are needed to ascertain causality. Proving cause and effect is much more difficult than just showing an association between two variables. For example, if it happened that everyone who had eaten a particular food became sick, and everyone who avoided that food remained well, then the food would clearly be associated with the sickness However, even if leftovers were found to be contaminated, it could not be proved that the food caused the sickness – unless all other possible causes (for example, environmental factors) had been ruled out.

Cerebrospinal fluid(CSF)

Clear fluid which is continuously being produced and absorbed by and in the brain, flowing in the ventricles (cavities) within the brain and around the surface of the brain and spinal cord

CSF otorrhea

Escape of CSF from the brain into the ear canal

Cervical spine

The cervical spine is the area of the vertebral column commonly refered to as the neck.

The cervical spine is made up of seven vertebrae, refered to by 'C', appended with an identifying number. The number indicates the level of the spine in which the particular vertebra is located. The top vertebra is C1, the lowest C7

Cervico-dorsal junction

The junction between the bottom of the cervical spine and the top of the dorsal (or thoracic) spine.

Clinical audit

A systematic process for setting and monitoring standards of clinical care. Whereas ‘guidelines’ define what the best clinical practice should be, ‘audit’ investigates whether best practice is being carried out. Clinical audit can be described as a cycle or spiral. Within the cycle there are stages that follow a systematic process of establishing best practice, measuring care against specific criteria, taking action to improve care, and monitoring to sustain improvement. The spiral suggests that as the process continues, each cycle aspires to a higher level of quality.

Clinical decision rule

A clinical decision rule/clinical prediction rule is generated by initially examining, and ultimately combining, a number of variables to predict the likelihood of a current diagnosis of a future event. Sometimes, if the likelihood is sufficiently high or low, the rule generates a suggested course of action1.

Clinical effectiveness

The extent to which a specific treatment or intervention, when used under usual or everyday conditions, has a beneficial effect on the course or outcome of disease compared to no treatment or other routine care. (Clinical trials that assess effectiveness are sometimes called management trials.) Clinical ‘effectiveness’ is not the same as efficacy.

Clinical impact

The effect that a guideline recommendation is likely to have on the treatment, or treatment outcomes, of the target population

Clinical question

This term is sometimes used in guideline development work to refer to the questions about treatment and care that are formulated in order to guide the search for research evidence. When a clinical question is formulated in a precise way, it is called a focused question.

Clinical trial

A research study conducted with patients which tests out a drug or other intervention to assess its effectiveness and safety. Each trial is designed to answer scientific questions and to find better ways to treat individuals with a specific disease. This general term encompasses controlled clinical trials and randomised controlled trials.


A healthcare professional providing patient care, for example, doctor, nurse, physiotherapist.

Closed head injury

A blow to the head or a severe shaking causing tearing, shearing or stretching of the nerves at the base of the brain, blood clots in or around the brain or oedema (swelling) of the brain. There is no penetration of the skull or brain tissue by an object; the skull may be fractured but this does not result in a direct connection between the brain and the outside. (see Penetrating Brain Injury)

Cluster randomisation

A study in which groups of individuals (for example, patients in a General Practitioner surgery or on a hospital ward) are randomly allocated to treatment groups. Take, for example, a smoking cessation study of two different interventions – leaflets and teaching sessions. Each General Practitioner surgery within the study would be randomly allocated to administer one of the two interventions. See also Cluster, Cluster design.


A condition affecting the blood’s ability to form a clot.

Cochrane Collaboration

An international organisation in which people find, appraise and review specific types of studies called randomised controlled trials. The Cochrane Database of Systematic Reviews contains regularly updated reviews on a variety of health issues and is available electronically as part of the Cochrane Library.

Cochrane Library

The Cochrane Library consists of a regularly updated collection of evidence-based medicine databases including the Cochrane Database of Systematic Reviews (reviews of randomised controlled trials prepared by the Cochrane Collaboration). The Cochrane Library is available on CD-ROM and the Internet.


A group of people sharing some common characteristic (for example, patients with the same disease), followed up in a research study for a specified period of time.

Cohort study

An observational study that takes a group (cohort) of patients and follows their progress over time in order to measure outcomes such as disease or mortality rates and make comparisons according to the treatments or interventions that patients received. Thus within the study group, subgroups of patients are identified (from information collected about patients) and these groups are compared with respect to outcome, for example, comparing mortality between one group that received a specific treatment and one group which did not (or between two groups that received different levels of treatment). Cohorts can be assembled in the present and followed into the future (a ‘concurrent’ or ‘prospective’ cohort study) or identified from past records and followed forward from that time up to the present (a ‘historical’ or ‘retrospective’ cohort study). Because patients are not randomly allocated to subgroups, these subgroups may be quite different in their characteristics and some adjustment must be made when analysing the results to ensure that the comparison between groups is as fair as possible.


A sleep-like state in which a person is not conscious.


Co-existence of a disease or diseases in the people being studied in addition to the health problem that is the subject of the study.

Community health services

General Practice, ambulance crews, NHS walk-in centres and dental practitioners.


The common result of a blow to the head or sudden deceleration usually causing an altered mental state, either temporary or prolonged. Physiological and/or anatomical disruption of connections between some nerve cells in the brain may occur. Often used by the public to refer to a brief loss of consciousness.

Confidence interval

A way of expressing certainty about the findings from a study or group of studies, using statistical techniques. A confidence interval describes a range of possible effects (of a treatment or intervention) that are consistent with the results of a study or group of studies. A wide confidence interval indicates a lack of certainty or precision about the true size of the clinical effect and is seen in studies with too few patients. Where confidence intervals are narrow they indicate more precise estimates of effects and a larger sample of patients studied. It is usual to interpret a ‘95%’ confidence interval as the range of effects within which we are 95% confident that the true effect lies.

Confounder or confounding factor

Something that influences a study and can contribute to misleading findings if it is not understood or appropriately dealt with. For example, if a group of people exercising regularly and a group of people who do not exercise have an important age difference then any difference found in outcomes about heart disease could well be due to one group being older than the other rather than due to the exercising. Age is the confounding factor here and the effect of exercising on heart disease cannot be assessed without adjusting for age differences in some way.


An alert cognitive state in which you are aware of yourself and your situation

Consensus development conference

A technique used for the purpose of reaching an agreement on a particular issue. It involves bringing together a group of about 10 people who are presented with evidence by various interest groups or experts who are not part of the decision making group. The group then retires to consider the questions in the light of the evidence presented and attempts to reach a consensus. See also Consensus methods.

Consensus methods

A variety of techniques that aim to reach an agreement on a particular issue. Formal consensus methods include Delphi and nominal group techniques, and consensus development conferences. In the development of clinical guidelines, consensus methods may be used where there is a lack of strong research evidence on a particular topic.


The extent to which the conclusions of a collection of studies used to support a guideline recommendation are in agreement with each other. See also Homogeneity.

Control group

A group of patients recruited into a study that receives no treatment, a treatment of known effect, or a placebo (dummy treatment) - in order to provide a comparison for a group receiving an experimental treatment, such as a new drug.

Controlled clinical trial (CCT)

A study testing a specific drug or other treatment involving two (or more) groups of patients with the same disease. One (the experimental group) receives the treatment that is being tested, and the other (the comparison or control group) receives an alternative treatment, a placebo (dummy treatment) or no treatment. The two groups are followed up to compare differences in outcomes to see how effective the experimental treatment was. A CCT where patients are randomly allocated to treatment and comparison groups is called a randomised controlled trial.

Cost benefit analysis

A type of economic evaluation where both costs and benefits of healthcare treatment are measured in the same monetary units. If benefits exceed costs, the evaluation would recommend providing the treatment.

Cost effectiveness

A type of economic evaluation that assesses the additional costs and benefits of doing something different. In cost effectiveness analysis, the costs and benefits of different treatments are compared. When a new treatment is compared with current care, its additional costs divided by its additional benefits is called the cost effectiveness ratio. Benefits are measured in natural units, for example, cost per additional heart attack prevented.

Cost utility analysis

A special form of cost effectiveness analysis where benefit is measured in quality adjusted life years. A treatment is assessed in terms of its ability to extend or improve the quality of life.


Pertaining to the cranium.

Craniocervical juntion

The junction between the base of the skull and the top of the cervical spine.

Crossover study design

A study comparing two or more interventions in which the participants, upon completion of the course of one treatment, are switched to another. For example, for a comparison of treatments A and B, half the participants are randomly allocated to receive them in the order A, B and half to receive them in the order B, A. A problem with this study design is that the effects of the first treatment may carry over into the period when the second is given. Therefore a crossover study should include an adequate ‘wash-out’ period, which means allowing sufficient time between stopping one treatment and starting another so that the first treatment has time to wash out of the patient’s system.

Cross-sectional study

The observation of a defined set of people at a single point in time or time period – a snapshot. (This type of study contrasts with a longitudinal study which follows a set of people over a period of time.)

Data set

A list of required information relating to a specific disease.

Decision analysis

A systematic way of reaching decisions, based on evidence from research. This evidence is translated into probabilities, and then into diagrams or decision trees which direct the clinician through a succession of possible scenarios, actions and outcomes.

Diagnostic study

A study to assess the effectiveness of a test or measurement in terms of its ability to accurately detect or exclude a specific disease.

Double blind study

A study in which neither the subject (patient) nor the observer (investigator/clinician) is aware of which treatment or intervention the subject is receiving. The purpose of blinding is to protect against bias.


A state of impaired awareness associated with a desire or inclination to sleep.

Dura Mater

The thick lining of the brain and spinal cord

Economic evaluation

Comparative analysis of alternative courses of action in terms of both their costs and consequences.


See Clinical effectiveness.


The extent to which a specific treatment or intervention, under ideally controlled conditions (for example, in a laboratory), has a beneficial effect on the course or outcome of disease compared to no treatment or other routine care.


Name for clinical procedures that are regarded as advantageous to the patient but not urgent.

Emergency Department

A clinical department in a district general or teaching hospital which has trained staff and equipment able to receive, resuscitate, investigate and initially manage the full spectrum of emergencies. Most units accept patients of all ages, some are restricted to adults, others to children. All should be open at all times and all its facilities should be available at all times.

Emergency Department Clinician

A medically qualified member of an emergency department or an appropriately trained nurse working in an emergency department.


Based directly on experience (observation or experiment) rather than on reasoning alone.


Study of diseases within a population, covering the causes and means of prevention.

European Paediatric Life Support course (EPLS)

The EPLS provider course is intended to provide training for multi-disciplinary healthcare professionals in the early recognition of the child in respiratory or circulatory failure and the development of the knowledge and core skills required to intervene to prevent further deterioration towards respiratory or cardiorespiratory arrest. (see

Event rate

The proportion of patients in a group for whom a specified health event or outcome is observed. Thus, if out of 100 patients, the event is observed in 27, the event rate is 0.27 or 27%. Control Event Rate (CER) and Experimental Event Rate (EER) are the terms used in control and experimental groups of patients respectively.

Evidence based clinical practice

Evidence based clinical practice involves making decisions about the care of individual patients based on the best research evidence available rather than basing decisions on personal opinions or common practice (which may not always be evidence based). Evidence based clinical practice therefore involves integrating individual clinical expertise and patient preferences with the best available evidence from research

Evidence table

A table summarising the results of a collection of studies which, taken together, represent the evidence supporting a particular recommendation or series of recommendations in a guideline.

Exclusion criteria

See Selection criteria.

Experimental study

A research study designed to test if a treatment or intervention has an effect on the course or outcome of a condition or disease - where the conditions of testing are to some extent under the control of the investigator. Controlled clinical trial and randomised controlled trial are examples of experimental studies.

Experimental treatment

A treatment or intervention (for example, a new drug) being studied to see if it has an effect on the course or outcome of a condition or disease.

External validity

The degree to which the results of a study hold true in non-study situations, for example, in routine clinical practice. May also be referred to as the generalisability of study results to non-study patients or populations.

Extradural (or epidural) haemorrage

A collection of blood between the skull inner surface and the dura mater caused by damage to the blood vessels running on the surface of the dura mater – often associated with a fracture of the skull. The underlying brain injury may not be severe initially but the increasing pressure caused by the bleeding inflicts further damage.

Extradural space

The space on the outer side of the dura mater.


The application of research evidence based on studies of a specific population to another population with similar characteristics.

Focal Neurological Deficit

A neurological deficit restricted to a particular part of the body or a particular activity

Forest plot

A graphical display of results from individual studies on a common scale, allowing visual comparison of results and examination of the degree of heterogeneity between studies.

Funnel plot

Funnel plots are simple scatter plots on a graph. They show the treatment effects estimated from separate studies on the horizontal axis against a measure of sample size on the vertical axis. Publication bias may lead to asymmetry in funnel plots.


The extent to which the results of a study hold true for a population of patients beyond those who participated in the research. See also External validity.

Glasgow Coma Scale

A standardised system used to assess the degree of brain impairment and to identify the seriousness of injury in relation to outcome. The system involves three determinants: eye opening, verbal responses and motor response all of which are evaluated independently according to a numerical value that indicates the level of consciousness and degree of dysfunction.

Gold standard

A method, procedure or measurement that is widely accepted as being the best available.


An accumulation of blood in or under the tissues.


A collection of blood in the middle ear space

Health economics

A field of conventional economics which examines the benefits of healthcare interventions (for example, medicines) compared with their financial costs.


Or lack of homogeneity. The term is used in meta-analyses and systematic reviews when the results or estimates of effects of treatment from separate studies seem to be very different – in terms of the size of treatment effects or even to the extent that some indicate beneficial and others suggest adverse treatment effects. Such results may occur as a result of differences between studies in terms of the patient populations, outcome measures, definition of variables or duration of follow-up.

Hierarchy of evidence

An established hierarchy of study types, based on the degree of certainty that can be attributed to the conclusions that can be drawn from a well conducted study. Well-conducted randomised controlled trials (RCTs) are at the top of this hierarchy. (Several large statistically significant RCTs which are in agreement represent stronger evidence than say one small RCT.) Well-conducted studies of patients’ views and experiences would appear at a lower level in the hierarchy of evidence.


This means that the results of studies included in a systematic review or meta analysis are similar and there is no evidence of heterogeneity. Results are usually regarded as homogeneous when differences between studies could reasonably be expected to occur by chance. See also Consistency.


Abnormally rapid breathing. Hyperventilation results in excessive intake of oxygen and increased elimination of carbon dioxide, which may eventually lead to a disturbance in the blood chemistry.


Abnormally low levels of glucose in the blood, leading to muscular weakness, confusion, sweating and, in severe cases, coma. Hypoglycaemia is a complication of many anti-diabetic treatments.

Inclusion criteria

See Selection criteria.


Aged under 1 year.

Intention to treat analysis

An analysis of a clinical trial where patients are analysed according to the group to which they were initially randomly allocated, regardless of whether or not they had dropped out, fully complied with the treatment, or crossed over and received the alternative treatment. Intention-to-treat analyses are favoured in assessments of clinical effectiveness as they mirror the non-compliance and treatment changes that are likely to occur when the treatment is used in practice.

Internal validity

Refers to the integrity of the study design.


Healthcare action intended to benefit the patient, for example, drug treatment, surgical procedure, psychological therapy, etc.

Interventional procedure

A procedure used for diagnosis or treatment that involves making a cut or hole in the patient’s body, entry into a body cavity or using electromagnetic radiation (including X-rays or lasers). The National Institute for Health and Clinical Excellence (NICE) has the task of producing guidance about whether specific interventional procedures are safe enough and work well enough for routine use.


Originating within the cranial (brain) cavity.

Intracranial haematoma

A collection of blood inside the cranium caused by damage to brain tissue or the rupture of a blood vessel. The resulting swelling can compress the brain.

Intracerebral haemorrhage

A bleed inside the brain tissue.

Intracranial lesion

A lesion of the brain.

Literature review

A process of collecting, reading and assessing the quality of published (and unpublished) articles on a given topic.

Longitudinal study

A study of the same group of people at more than one point in time. (This type of study contrasts with a cross sectional study which observes a defined set of people at a single point in time.)


The lower jaw as a functional unit, regardless of which bones or cartilage make up the lower jaw in a particular organism.


Stiffness of the neck associated with backwards extension of the cervical spine.

Meta analysis

Results from a collection of independent studies (investigating the same treatment) are pooled, using statistical techniques to synthesise their findings into a single estimate of a treatment effect. Where studies are not compatible for example, because of differences in the study populations or in the outcomes measured, it may be inappropriate or even misleading to statistically pool results in this way. See also Systematic review & Heterogeneity.


The overall approach of a research project, for example, the study will be a randomised controlled trial, of 200 people, over one year.

Methodological quality

The extent to which a study has conformed to recognised good practice in the design and execution of its research methods.

Monte Carlo simulation

A modelling technique that uses random numbers to capture the effects of uncertainty. Multiple simulations are run (usually somewhere between 1,000 and 10,000). For each simulation, the value of each uncertain variable in the analysis is selected at random from a probability distribution for the value of that variable. The simulation results are compiled, providing a probability distribution for the overall result.

Motor response

Movement in response to an external stimulus

Multicentre study

A study where subjects were selected from different locations or populations, for example, a co-operative study between different hospitals; an international collaboration involving patients from more than one country.

Negative predictive value

The proportion of individuals with a negative test result who do NOT have the disease.

Neurorehabilitation services

A programme of clinical and vocational services with the goal of returning brain injured patients to a satisfying occupation,.


A surgical specialty for the treatment of diseases and disorders of the brain, spinal cord and nerves.

Non-experimental study

A study based on subjects selected on the basis of their availability, with no attempt having been made to avoid problems of bias.

Non-systematic review

See Review.

Objective measure

A measurement that follows a standardised procedure which is less open to subjective interpretation by potentially biased observers and study participants.

Observational study

In research about diseases or treatments, this refers to a study in which nature is allowed to take its course. Changes or differences in one characteristic (for example, whether or not people received a specific treatment or intervention) are studied in relation to changes or differences in other(s) (for example, whether or not they died), without the intervention of the investigator. There is a greater risk of selection bias than in experimental studies.

Occipital condyle

The articulation point between the skull and the first cervical vertebra.

Odds ratio

Odds are a way of representing probability, especially familiar for betting. In recent years odds ratios have become widely used in reports of clinical studies. They provide an estimate (usually with a confidence interval) for the effect of a treatment. Odds are used to convey the idea of ‘risk’ and an odds ratio of 1 between two treatment groups would imply that the risks of an adverse outcome were the same in each group. For rare events the odds ratio and the relative risk (which uses actual risks and not odds) will be very similar. See also Relative risk, Risk ratio.


The end result of care and treatment and/or rehabilitation. In other words, the change in health, functional ability, symptoms or situation of a person, which can be used to measure the effectiveness of care/treatment/rehabilitation. Researchers should decide what outcomes to measure before a study begins; outcomes are then assessed at the end of the study.


Pertaining to children and infants


Abnormal sensation such as burning or tingling due to a disorder of the sensory nervous system.

Penetrating head injury

Head injury where an object penetrates the scalp and skull and enters the brain or its lining.

Performance bias

Systematic differences in care provided apart from the intervention being evaluated. For example, if study participants know they are in the control group they may be more likely to use other forms of care; people who know they are in the experimental group may experience placebo effects, and care providers may treat patients differently according to what group they are in. Masking (blinding) of both the recipients and providers of care is used to protect against performance bias.

Periorbital haemotoma

Bleeding around or behind the eyes.

Pilot study

A small scale ‘test’ of the research instrument. For example, testing out (piloting) a new questionnaire with people who are similar to the population of the study, in order to highlight any problems or areas of concern, which can then be addressed before the full scale study begins.


Placebos are fake or inactive treatments received by participants allocated to the control group in a clinical trial which are indistinguishable from the active treatments being given in the experimental group. They are used so that participants are ignorant of their treatment allocation in order to be able to quantify the effect of the experimental treatment over and above any placebo effect due to receiving care or attention.

Placebo effect

A beneficial (or adverse) effect produced by a placebo and not due to any property of the placebo itself.

Positive predictive value

The proportion of individuals with a positive test result who actually have the disease.


See Statistical power.

Primary care

Healthcare delivered to patients outside hospitals. Primary care covers a range of services provided by General Practitioners, nurses and other healthcare professionals, dentists, pharmacists and opticians.


How likely an event is to occur, for example, how likely a treatment or intervention will alleviate a symptom.

Prognostic factor

Patient or disease characteristics, for example, age or co-morbidity, which influence the course of the disease under study. In a randomised trial to compare two treatments, chance imbalances in variables (prognostic factors) that influence patient outcome are possible, especially if the size of the study is fairly small. In terms of analysis these prognostic factors become confounding factors. See also Prognostic marker.

Prognostic marker

A prognostic factor used to assign patients to categories for a specified purpose – for example, for treatment, or as part of a clinical trial, according to the likely progression of the disease. For example, the purpose of randomisation in a clinical trial is to produce similar treatment groups with respect to important prognostic factors. This can often be achieved more efficiently if randomisation takes place within subgroups defined by the most important prognostic factors. Thus if age was very much related to patient outcome then separate randomisation schemes would be used for different age groups. This process is known as stratified random allocation.

Prospective study

A study in which people are entered into the research and then followed up over a period of time with future events recorded as they happen. This contrasts with studies that are retrospective.

Publication bias

Studies with statistically significant results are more likely to get published than those with non-significant results. Meta-analyses that are exclusively based on published literature may therefore produce biased results. This type of bias can be assessed by a funnel plot.

P value

If a study is done to compare two treatments then the P value is the probability of obtaining the results of that study, or something more extreme, if there really was no difference between treatments. (The assumption that there really is no difference between treatments is called the ‘null hypothesis’.) Suppose the P-value was P=0.03. What this means is that if there really was no difference between treatments then there would only be a 3% chance of getting the kind of results obtained. Since this chance seems quite low we should question the validity of the assumption that there really is no difference between treatments. We would conclude that there probably is a difference between treatments. By convention, where the value of P is below 0.05 (that is, less than 5%) the result is seen as statistically significant. Where the value of P is 0.001 or less, the result is seen as highly significant. P values just tell us whether an effect can be regarded as statistically significant or not. In no way do they relate to how big the effect might be, for which we need the confidence interval.

Qualitative research

Qualitative research is used to explore and understand people’s beliefs, experiences, attitudes, behaviour and interactions. It generates non-numerical data, for example, a patient’s description of their pain rather than a measure of pain. In healthcare, qualitative techniques have been commonly used in research documenting the experience of chronic illness and in studies about the functioning of organisations. Qualitative research techniques such as focus groups and in depth interviews have been used in one-off projects commissioned by guideline development groups to find out more about the views and experiences of patients and carers.

Quality adjusted life years

A measure of health outcome. QALYS are calculated by estimating the total life-years gained from a treatment and weighting each year with a quality of life score.

Quantitative research

Research that generates numerical data or data that can be converted into numbers, for example clinical trials or the national Census which counts people and households.

Quasi experimental study

A study designed to test if a treatment or intervention has an effect on the course or outcome of disease. It differs from a controlled clinical trial and a randomised controlled trial in that:

  1. the assignment of patients to treatment and comparison groups is not done randomly, or patients are not given equal probabilities of selection, or
  2. the investigator does not have full control over the allocation and/or timing of the intervention, but nonetheless conducts the study as if it were an experiment, allocating subjects to treatment and comparison groups.

Random allocation or Randomisation

A method that uses the play of chance to assign participants to comparison groups in a research study, for example, by using a random numbers table or a computer-generated random sequence. Random allocation implies that each individual (or each unit in the case of cluster randomisation) being entered into a study has the same chance of receiving each of the possible interventions.

Randomised controlled trial

A study to test a specific drug or other treatment in which people are randomly assigned to two (or more) groups: one (the experimental group) receiving the treatment that is being tested, and the other (the comparison or control group) receiving an alternative treatment, a placebo (dummy treatment) or no treatment. The two groups are followed up to compare differences in outcomes to see how effective the experimental treatment was. (Through randomisation, the groups should be similar in all aspects apart from the treatment they receive during the study.)

Rehabilitation services

A programme of clinical and vocational services with the goal of returning patients to a satisfying occupation.

Relative risk

A summary measure which represents the ratio of the risk of a given event or outcome (for example, an adverse reaction to the drug being tested) in one group of subjects compared to another group. When the ‘risk’ of the event is the same in the two groups the relative risk is 1. In a study comparing two treatments, a relative risk of 2 would indicate that patients receiving one of the treatments had twice the risk of an undesirable outcome than those receiving the other treatment. Relative risk is sometimes used as a synonym for risk ratio.


Reliability refers to a method of measurement that consistently gives the same results. For example someone who has a high score on one occasion tends to have a high score if measured on another occasion very soon afterwards. With physical assessments it is possible for different clinicians to make independent assessments in quick succession – and if their assessments tend to agree then the method of assessment is said to be reliable.

Retrospective study

A retrospective study deals with the present/past and does not involve studying future events. This contrasts with studies that are prospective.


Summary of the main points and trends in the research literature on a specified topic. A review is considered non-systematic unless an extensive literature search has been carried out to ensure that all aspects of the topic are covered and an objective appraisal made of the quality of the studies.

Risk ratio

Ratio of the risk of an undesirable event or outcome occurring in a group of patients receiving experimental treatment compared with a comparison (control) group. The term relative risk is sometimes used as a synonym of risk ratio.


A part of the study’s target population from which the subjects of the study will be recruited. If subjects are drawn in an unbiased way from a particular population, the results can be generalised from the sample to the population as a whole.


Refers to the way participants are selected for inclusion in a study.

Sampling frame

A list or register of names which is used to recruit participants to a study.

Secondary care

Care provided in hospitals.


An uncontrolled discharge of nerve impulses which may spread throughout the brain. It usually lasts only a few minutes. It may be associated with loss of consciousness or loss of bowel and bladder control.

Selection bias

Selection bias has occurred if:

  • the characteristics of the sample differ from those of the wider population from which the sample has been drawn OR
  • there are systematic differences between comparison groups of patients in a study in terms of prognosis or responsiveness to treatment.

Selection criteria

Explicit standards used by guideline development groups to decide which studies should be included and excluded from consideration as potential sources of evidence.

Semi-structured interview

Structured interviews involve asking people pre-set questions. A semi-structured interview allows more flexibility than a structured interview. The interviewer asks a number of open-ended questions, following up areas of interest in response to the information given by the respondent.


In diagnostic testing, it refers to the chance of having a positive test result given that you have the disease. 100% sensitivity means that all those with the disease will test positive, but this is not the same the other way around. A patient could have a positive test result but not have the disease – this is called a ‘false positive’. The sensitivity of a test is also related to its ‘negative predictive value’ (true negatives) – a test with a sensitivity of 100% means that all those who get a negative test result do not have the disease. To fully judge the accuracy of a test, its Specificity must also be considered.


Plural of sequela, which is any abnormal condition that occurs subsequent to and/or is caused by disease, injury, or treatment.

Single blind study

A study in which either the subject (patient/participant) or the observer (clinician/investigator) is not aware of which treatment or intervention the subject is receiving.

Specific indication

When a drug or a device has a specific remit to treat a specific condition and is not licensed for use in treating other conditions or diseases.


In diagnostic testing, it refers to the chance of having a negative test result given that you do not have the disease. 100% specificity means that all those without the disease will test negative, but this is not the same the other way around. A patient could have a negative test result yet still have the disease – this is called a ‘false negative’. The specificity of a test is also related to its ‘positive predictive value’ (true positives) – a test with a specificity of 100% means that all those who get a positive test result definitely have the disease. To fully judge the accuracy of a test, its Sensitivity must also be considered.

Standard deviation

A measure of the spread, scatter or variability of a set of measurements. Usually used with the mean (average) to describe numerical data.

Stand by call

Contact between a paramedic or other healthcare worker and an emergency department, by telephone or radio, to alert the department to the impending arrival of a seriously ill or injured patient who will require immediate resuscitation.

Statistical power

The ability of a study to demonstrate an association or causal relationship between two variables, given that an association exists. For example, 80% power in a clinical trial means that the study has a 80% chance of ending up with a P value of less than 5% in a statistical test (that is, a statistically significant treatment effect) if there really was an important difference (for example, 10% versus 5% mortality) between treatments. If the statistical power of a study is low, the study results will be questionable (the study might have been too small to detect any differences). By convention, 80% is an acceptable level of power. See also P value.

Structured interview

A research technique where the interviewer controls the interview by adhering strictly to a questionnaire or interview schedule with pre-set questions.

Study checklist

A list of questions addressing the key aspects of the research methodology that must be in place if a study is to be accepted as valid. A different checklist is required for each study type. These checklists are used to ensure a degree of consistency in the way that studies are evaluated.

Study population

People who have been identified as the subjects of a study.

Study quality

See Methodological quality.

Study type

The kind of design used for a study. Randomised controlled trial, case-control study, cohort study are all examples of study types.

Sub-group analysis

An analysis in which the intervention effect is evaluated in a defined subset of the participants in the trial, or in complementary subsets, such as by sex or in age categories.

Subdural space

The space beneath the dura mater, between it and the much thinner arachnoid mater. This is often the area of rupture of delicate thin-walled veins following head injuries.

Subdural haematoma (or haemorrhage)

A collection of blood between the dura mater and the arachnoid mater caused by traumatic damage to the associated brain and blood vessels. The rise in pressure caused by such bleeding can cause further significant damage


A person who takes part in an experiment or research study.


A partial dislocation of a joint in which the joint surfaces remain in contact, albeit out of alignment.


A study in which information is systematically collected from people (usually from a sample within a defined population).


Methodical, according to plan; not random.

Systematic error

Refers to the various errors or biases inherent in a study. See also Bias.

Systematic review

A review in which evidence from scientific studies has been identified, appraised and synthesised in a methodical way according to predetermined criteria. May or may not include a meta-analysis.


Involving the whole body.

Target population

The people to whom guideline recommendations are intended to apply. Recommendations may be less valid if applied to a population with different characteristics from the participants in the research study – for example, in terms of age, disease state, social background.

Tertiary centre

A specialist medical centre providing complex treatments which receives referrals from both primary and secondary care. Sometimes called a tertiary referral centre. See also Primary care and Secondary care.


Involuntary spasms of the musculature in the neck.


Use of three or more different research methods in combination; principally used as a check of validity. The more the different methods produce similar results, the more valid the findings.

Triple blind study

A study in which the statistical analysis is carried out without knowing which treatment patients received, in addition to the patients and investigators/clinicians being unaware which treatment patients were getting.


A temporary or prolonged loss of awareness of self and of surroundings


Assessment of how well a tool or instrument measures what it is intended to measure. See also External validity, Internal validity.


A measurement that can vary within a study, for example, the age of participants. Variability is present when differences can be seen between different people or within the same person over time, with respect to any characteristic or feature which can be assessed or measured.

Copyright © 2007, National Collaborating Centre for Acute Care.

Apart from any fair dealing for the purposes of research or private study, criticism or review, as permitted under the Copyright, Designs and Patents Act, 1988, no part of this publication may be reproduced, stored or transmitted in any form or by any means, without the prior written permission of the publisher or, in the case of reprographic reproduction, in accordance with the terms of licences issued by the Copyright Licensing Agency in the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publisher at the UK address printed on this page.

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Bookshelf ID: NBK53056


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