U.S. flag

An official website of the United States government

NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.

National Research Council (US) and Institute of Medicine (US) Committee on Drug Use in the Workplace; Normand J, Lempert RO, O'Brien CP, editors. Under the Influence? Drugs and the American Work Force. Washington (DC): National Academies Press (US); 1994.

Cover of Under the Influence?

Under the Influence? Drugs and the American Work Force.

Show details

IIntroduction

Drug use and drug abuse are not new problems, but rather are ones that receive heightened attention at various points in time. The past 200 years have witnessed the gin epidemic in England, the opium wars in Asia, and the woefully forgotten cocaine patent medicine tragedies at the turn of this century. More recently, we have seen the end of prohibition in the 1930s, ''reefer madness" in the 1940s, the drug culture of the 1960s, and the heroin epidemic of the 1970s. Currently—and for the second time in this century—cocaine has become a major problem in this country, particularly in its newest and more virulent form of rock or crack cocaine. The use of illegal drugs in recent years is thought to pose problems so severe as to justify a "war on drugs."

The current war on drugs overlooks, however, the abuse of alcohol and tobacco, which cause more deaths in the United States than all illegal drugs combined (Newcomb, 1992). Whereas illegal drugs are estimated to be responsible for approximately 30,000 premature deaths in the United States per year (Reuter, 1992), tobacco is responsible for nearly 400,000 premature deaths per year, and alcohol accounts for nearly 100,000 fatalities per year (Julien, 1992).

Drug use, and more specifically alcohol use, by U.S. workers has a long history. Fillmore (1984) argues that the acceptability of drug use on the job changed as a result of the industrial revolution and the temperance movement. She points out (p. 41) that "… prior to the industrial revolution, work and drinking appear to have been inseparable in the U.S. Although the drunken worker was disapproved, especially among the Puritans, drinking in the workplace was considered normal behavior. Drinking was long associated with hard work." The condemnation of alcohol and drug use in the workplace is a relatively recent reaction, arising largely over the past 100 years. Ames (1989), in her historical review of the influence of alcohol-related movements on drinking in the American workplace, points out, however, that many workplaces have been slow to integrate policies that reflect changes in society's views of alcohol. Indeed, in the past year, employees of a well-known Washington hotel were picketing the establishment over the management's attempt to abolish their contractual right to consume three beers per shift.

Recently, there has been increased awareness of the costs related to alcohol and drug abuse on the job. This may be due, at least in part, to the publicity given two recent tragic accidents, the Exxon Valdez oil spill in Alaska and the Amtrak train crash in Maryland, as well as to the "war on drugs" policies of the Reagan and Bush administrations. The effects of such events on perceptions of the alcohol and other drug abuse problem remind us that what constitutes unacceptable drug use varies not only across cultures but also within a culture. The perception of illicit drug use as a significant social problem in contemporary America is the product of social attitudes and perceptions that are shaped by policy makers and others in influential positions (Humphreys and Rappaport, 1993). Yet this does not mean that the problems associated with drug use lack reality. Drug use can have a devastating effect on the quality of life for both individuals and society. The problems associated with drug use, both licit and illicit, are complex and varied. They are not amenable to quick and easy solutions, for example, by simple slogans or by the appointment of a new drug czar.

A few scenarios illustrate the diversity of the problems associated with drug use by the U.S. work force:

John's boss wanted him to make the sale. If the company could unload those extra parts, perhaps they could forestall a deficit for the quarter. John was given a liberal expense account; he was to wine and dine the prospective buyers before talking business. The restaurant was top of the line, and the liquor, expensive. When John woke up the next morning, he couldn't remember whether he made the sale, the terms of the bargain, or even how he got home.

Hazel had worked hard to get where she was. She worked days and took cosmetology courses at night. She then worked in her girlfriend's beauty shop and finally was able to save and borrow enough to open her own shop. After three years, she had a thriving business. Hazel smoked marijuana from time to time, and it caused her no problems. So when she was offered cocaine by her cousin, she thought nothing of it. Within six months, she was using cocaine regularly; within a year, she was using it in the back of the shop and taking money from the cash register to buy more; within a year and a half, she filed for bankruptcy.

George, an independent trucker, borrowed money to buy his own rig and now does transcontinental hauling. The longer he drives, the more money he makes to pay off those loans. Coffee, caffeine pills, and diet pills often allow him to drive 18 to 20 hours at a stretch, but they also make him an unsafe driver. Seventeen hours after leaving a truckstop, he ran a van off the road, killing three of its occupants.

Dr. S., a neurosurgical resident, was looking forward to a weekend off, especially since the past week had been stressful with difficult cases and learning new procedures. A former medical school classmate was coming to visit, and they had planned to relive the good old days by going out on the town. Unfortunately, the doctor assigned to take calls became acutely ill, and Dr. S. was forced to carry the emergency pager. Although a bit angered by this turn of events, Dr. S. did not let it spoil his plans. After having picked up his old buddy at the airport, they headed for the bar district. At the second bar, after his third beer, the pager went off. There was an auto accident, and Dr. S. was needed immediately at the hospital.

Sally felt a bit sick; she had a bad headache, nausea, and a mild tremor. She knew that she and Jim had too much to drink last night, but it was a great party, and he would be away for two weeks. Although she wanted to call in sick for work, she had been told that, if she called in one more time, she would be fired. She knew that a few cups of coffee would pull her together, and she felt relieved that she had to drive the school bus only for a couple of hours before returning home to get some sleep.

These scenarios are not intended to convey the idea that using alcohol or other drugs necessarily has negative consequences, but rather to illustrate the diversity in the nature, magnitude, and potential severity of the problems associated with alcohol and other drug use. The problems are serious, but there is no point in overstating or sensationalizing the difficulties associated with alcohol and other drug use in the workplace or by the U.S. work force. If we are to make wise policy decisions, we should be aware that much alcohol and other drug use consists of infrequent or moderate use that does not necessarily result in harmful consequences, and that most illicit drug-using careers are short-lived and end without requiring any treatment (Gerstein and Harwood, 1990).

The vignettes help to distinguish between two different perspectives concerning the consequences of alcohol and other drug use or abuse: (1) a concern for public health and social welfare and (2) a concern for workplace productivity. From a public health/social welfare perspective, the costs associated with drug abuse are exorbitant. They include the health care costs for treating AIDS patients, crack babies, victims of premature heart attacks, and others. They also include costs related to missed developmental opportunities that are linked to premature births, child abuse, and the like. Society must also absorb the costs of criminal drug activities (e.g., theft, court overload, incarceration) as well as the emotional and physical costs of the violence that often accompanies the trade in and use of illicit drugs. Researchers have estimated that illicit drug abuse results in the loss of approximately 38 years of life due to premature death (Rice et al., 1990).1

From the workplace perspective, productivity losses, employers' health costs, and workplace accidents are the principal harm caused by the use of alcohol and other drugs. These costs may result from alcohol and other drug use on or off the job and can be devastating to particular businesses.

This report is concerned with the implications of drug use for workplace safety and productivity. It looks at the prevalence of alcohol and other drug use by the U.S. work force, the impact of alcohol and other drug use on job-related behavior, and the effectiveness of workplace drug intervention programs. This emphasis on workplace outcomes rather than social consequences more generally affects the purpose, methods, and evaluation criteria used in this report and by researchers investigating these issues.

The Committee's Charge

The committee was charged with: (1) analyzing the available research knowledge on the prevalence and etiology of drug consumption by the work force; (2) studying the impact of drug behavior on work performance, productivity, safety, and health; and (3) evaluating the effectiveness, costs, and benefits of organizational drug intervention programs at the work site.

Three key terms in the committee's charge require clarification, because their definitions have a significant bearing on the committee's interpretation of the scope of its work. The first term is drug: how we define this term determines which substances are considered in the report. The second term is use: how the committee defines this term affects the degree of drug involvement considered (i.e., use, abuse, or dependence). And the third term is work force: how the committee defines this term delineates the population of interest for this study.

Definition Of The Term Drug

President Reagan's Executive Order 12564 in September 1986, which required all federal agencies to develop programs and policies to achieve a drug-free federal workplace, necessitated an agreed-on definition of the term drugs. Given the political climate at that time, the implication that drugs meant illicit drugs was clear. Consequently, in 1988 the U.S. Department of Health and Human Services (HHS) "Mandatory Guidelines for Federal Workplace Drug Testing Programs" limited the number of drugs to be tested for to the following commonly used illicit drug classes: (1) marijuana, (2) opiates (heroin, morphine), (3) cocaine, (4) amphetamine and methamphetamine, and (5) phencyclidine.

Of course, many other drugs in addition to these five groups are misused or abused, and, from the committee's point of view, this list is too narrow. As Dubowski and Tuggle (1990:74) argue: "If one … postulates that drug-use testing in its common current context is intended to identify and ultimately to eliminate or at least limit hazards to persons, operations, property, and the public at large arising from inappropriate use of drugs by the workforce, it becomes clear that the primary targets of the testing efforts should be those drugs that have mood-altering properties as either primary or secondary characteristics. Whether this mood-altering substance is licit or illicit should not be a factor." Consistent with this argument, Alleyne et al. (1991) find that, among work-related accident victims, alcohol is the most common drug involved in occupational fatalities, with 11 percent of industrial accident victims having alcohol in their system. This estimate is consistent with figures reported in earlier studies (Baker et al., 1982; Lewis and Cooper, 1989). In the trucking industry, a recent report from the National Transportation Safety Board (1990) showed that, for truck driver fatalities, the most prevalent drugs detected were alcohol and marijuana (both at a rate of 13 percent). Given that alcohol is considered the most commonly abused substance—from both the public health/social welfare and workplace perspectives—any intervention program that strives to have a meaningful impact on either the health of the general population or the productivity of a specific work force should not limit its efforts to the detection of a few illicit drug classes but should target alcohol as well.2

At the opposite end of the spectrum, some illicit drugs (such as LSD or synthesized substituted amphetamines) are used by so few individuals that they are unlikely to pose significant problems in the work force. The use of such drugs, however, may pose a risk under certain conditions and situations. For example, methamphetamine, which represented less than 2 percent of emergency room cases nationwide in 1988, accounted for 27 percent of emergency room cases in San Diego during that year (National Institute on Drug Abuse, 1989); methamphetamine might therefore have been a serious concern to the local business community in San Diego in 1988.

Although the committee judged that a consideration of prescribed medications was beyond the scope of its study, a short discussion is warranted. There are no accurate estimates of the prevalence of prescribed medications used by U.S. workers, but the fact that approximately 1.6 billion prescriptions are filled annually by outpatient pharmacies (U.S. Public Health Service, 1992) suggests that a substantial number of workers are using or are under the effect of therapeutic drugs while at work. Sales of over-the-counter medications that might be expected to affect safety or productivity (e.g., antihistamines, medication with significant alcohol content) run into the billions, which suggests that these drugs may also affect the workplace. More important, there is little scientific knowledge concerning the effects of medication on work productivity and safety (DeHart, 1990). Most of the scientific information on the effects of those drugs on performance have come from controlled laboratory studies, which are reviewed in Chapter 4 of this report. As pointed out in a recent issue of the Journal of Occupational Medicine (April 1990), better monitoring of the incidence of adverse effects of therapeutic drugs on workplace performance is needed (Tilson, 1990), as well as a better understanding of the interactive effects of the pharmacokinetics of therapeutic drugs, work activities, and the environment (DeHart, 1990) and proper evaluations of the interactive effects of various therapeutic drugs and worker performance (especially with the older worker). Nonetheless, as Potter (1990) concludes, one should keep in mind that, for many conditions, the lack of treatment is likely to cause more performance impairments than the side effects of such treatment.

Based on these considerations, the committee defines the term drug to include any psychotropic substance that, if consumed, will affect a person's psychological status or physiological state or behavior. We consider only substances whose use is problematic enough to represent a meaningful threat to the welfare of individual users or others and whose prevalence is high enough among the work force to have the potential to affect business productivity. The report focuses its attention on general drug class categories (see Table 1.1), includes alcohol within its scope, and briefly addresses issues surrounding tobacco.

TABLE 1.1. Drug Classes.

TABLE 1.1

Drug Classes.

Definitions of Use, Abuse, and Dependence

Drug taking can be classified into one of three categories: (1) use, (2) abuse, and (3) dependence. Use is defined as the limited, controlled consumption of a drug (in terms of frequency and quantity) without significant toxic, adverse physical, or psychological consequences to the user (Glantz, 1992). Regular use of prescribed medications, legal drugs such as nicotine, caffeine, and alcohol, and certain illegal drugs can lead to physiological dependence. This simply means that the abrupt cessation of drug taking produces a set of symptoms called a withdrawal syndrome. The presence of physiological dependence does not necessarily imply abuse or dependence in the behavioral sense. Abuse is defined as a level of drug use that typically leads to adverse consequences (physical or psychological). Drug use at this level is not necessarily associated with any particular frequency but is associated with use in quantities sufficient to result in some toxicity to the user, and the patterns of use usually have some characteristics of psychopathological behavior. Dependence in the behavioral sense is defined as a level of drug use that has significant adverse physical and psychological consequences. This level of use is characterized by the consumption of toxic doses of the drug that impair the user's ability to function and is also characterized by a compulsive desire to use a drug repeatedly.

Dependence is associated with an overwhelming involvement in drugseeking and drug-taking activities. It typically leads to drug tolerance, which means that increasing doses of the drug are needed to obtain the same physiological effect. Each of these terms describe points along a continuum of drug involvement. Of those who use a drug, some never try it again, others continue their use on an irregular or regular basis and take it in doses that amount to abuse, and some continue to the stage of addiction and dependence.3 As depicted in Chapter 3 of this report, with the exception of users of nicotine and caffeine which are associated with physiological dependence, most workers who have taken drugs can be classified as users of alcohol or other drugs, rather then as abusers or as individuals who are dependent (in either sense) on them.

The clinical diagnostic criteria used to determine an individual's location on the use-dependence continuum are to some extent subjective—the diagnostic process is open to influences from culture, society, and individual diagnosticians. Furthermore, applications of these definitions can vary according to the type of substance taken, the dose taken, the route of administration, prior health history, vulnerability factors, and the environmental context of use. It is safe to say, however, that in the workplace, with the exception of nicotine and caffeine (which are associated with dependence), the majority of those experienced with drugs are users of alcohol and other drugs, rather than abusers or dependent. Thus, when we refer without additional qualification to alcohol or other drug use among the work force, we mean to encompass any degree of drug use, ranging from casual use to dependence. Whenever appropriate we acknowledge the special problems caused by abuse and dependence.

Definition of the Work Force

Although one might confine the question of alcohol and other drug use by the work force to the use of those substances by employees while at work, the committee believes its charge requires a more encompassing definition. By work force we mean to include any active member of the labor force, including those seeking or available for employment. Work force alcohol and other drug use is the use of those substances by any work force member, whether the use occurs on or off the job, so long as the use has potential workplace effects. Consequently, issues concerning hangover or residual effects of alcohol and other drugs taken when not at work, as well as correlates of individual alcohol and other drug use and work force participation, are all relevant.

Scope Of The Report

This report examines a wide range of studies concerning the magnitude and severity of alcohol and other drug use among the work force as well as the probable effects of alcohol and other drugs on the work force. These studies include laboratory experiments, field studies, epidemiological surveys, organization-specific prevalence reports, and descriptive accounts as well as empirical evaluations of drug intervention programs. As is typical in other areas of behavioral research, the results of no single study are definitive (Lempert, 1989). Consider, for example, studies of the effects of drug testing on employee job performance. Studies examining this question suffer from a variety of flaws, including inadequate samples, problematic measures, and incomplete analyses. But the flaws differ across studies, so that when the results of a number of studies converge, the convergent results are likely to be valid. For this reason, whenever possible in this report, we ground our conclusions in the accumulated results of multiple studies. In some areas, however, this was impossible and the committee's efforts to answer certain questions were hampered by the lack of adequately sound research.

There are a number of mechanisms that are used by the scientific community to ensure the scientific adequacy of studies, including the peer review process used in scientific and professional journals, the use of scientific advisory panels and review groups, and the use of external reviews of research prior to its execution and/or publication. Many of the studies reviewed in this report failed to incorporate any of these mechanisms, and they frequently failed to employ the research methods most likely to yield interpretable and valid results or failed to report their results in ways that allowed us to adequately evaluate their findings. One conclusion we reached after examining the body of relevant research was that much of it was methodologically weak, and that future research in this area should incorporate such mechanisms as peer review, which will help improve the quality of both the research itself and the research reports that appear in the scientific literature on the effects of alcohol and other drug use on the U.S. work force.

This report is organized into three parts. Following this introduction, Part I first summarizes current knowledge concerning the primary causes and the severity and magnitude of alcohol and other drug use. Chapter 2 provides some insight into the etiology of alcohol and other drug use. Chapter 3 examines the latest prevalence estimates and trends in alcohol and other drug use behavior.

Part II addresses the critical issue of the impact of alcohol and other drug use on behavior. Chapter 4 summarizes a substantial body of literature that has evaluated the effects of various classes of drugs on performance within controlled laboratory settings, including effects of stimulants (e.g., amphetamines, cocaine), sedatives (e.g., benzodiazepines, alcohol), and marijuana. Chapter 5 reviews applied research on the potential causal relationship between alcohol and other drug use and various work-related outcome measures, such as job performance and productivity indicators.

Part III addresses the effectiveness of intervention programs with special emphasis given to drug-testing programs. Given that drug-testing programs are commonplace in American corporations today, the committee felt that special attention to this form of drug use intervention program was imperative.4 Chapter 6 describes current analytical methods used to test biologic specimens for drugs and discusses the strengths and weaknesses associated with the procedures and techniques for analyzing biological specimens (e.g., urine, hair, saliva). It also reviews indirect methods of assessing drug use, including attitude questionnaires and others. Chapter 7 provides a critical review of studies that have attempted to evaluate the effectiveness of workplace drug-testing programs. Chapter 8 reviews the scientific evidence on the effectiveness of employee assistance programs (EAPs) and workplace prevention programs.

Appendix A addresses basic but critical measurement, methodological, and design issues, and Appendix B addresses relevant drug-testing legal issues.

References

  • Alleyne, B.C., P. Stuart, and R. Copes 1991. Alcohol and other drug use in occupational fatalities. Journal of Occupational Medicine 33(4):496-500. [PubMed: 2037904]
  • American Psychiatric Association 1987. Diagnostic and Statistical Manual of Mental Disorders, 3rd ed., revised. Washington, D.C.: American Psychiatric Association.
  • Ames, G.M. 1989. Alcohol-related movements and their effects on drinking policies in the American workplace: an historical review. The Journal of Drug Issues 19(4):489-510.
  • Babor, T.F. 1992. Nosological considerations in the diagnosis of substance use disorders. Pp. 53-74 in M. Glantz, editor; and R. Pickens, editor. , eds., Vulnerability to Drug Abuse. Washington, D.C.: American Psychological Association.
  • Baker, S., J.S. Samkoff, R.S. Fisher, et al. 1982 Fatal occupational injuries. Journal of the American Medical Association 248:692-697. [PubMed: 7097919]
  • DeHart, R.L. 1990. Medication and the work environment. Journal of Occupational Medicine 32(4):310-312. [PubMed: 2335796]
  • Dubowski, K.M., and R.S. Tuggle, III 1990. Drug-Use Testing in the Workplace: Law and Science. Eau Claire, Wis.: Professional Education Systems, Inc.
  • Fillmore, K.M. 1984. Research as a handmaiden of policy: an appraisal of estimates of alcoholism and its cost in the workplace. Journal of Public Health Policy March:40-64. [PubMed: 6201509]
  • Gerstein, D.R., and H.J. Harwood 1990. Treating Drug Problems, Vol. 1. Washington, D.C.: National Academy Press.
  • Glantz, M.D. 1992. A developmental psychopathology model of drug abuse vulnerability. Pp. 389-418 in M. Glantz, editor; and R. Pickens, editor. , eds., Vulnerability to Drug Abuse. Washington, D.C.: American Psychological Association.
  • Hughes, J. 1992. Clinical importance of caffeine withdrawal—editorial. New England Journal of Medicine 327(16):1160-1161. [PubMed: 1528212]
  • Humphreys, K., and J. Rappaport 1993. From the community mental health movement to the war on drugs. American Psychologist 48(8):892-901. [PubMed: 8379597]
  • Journal of Occupational Medicine 1990. Medication-induced performance decrements. Journal of Occupation Medicine 32(4):309-370.
  • Julien, R.M. 1992. A Primer of Drug Action, 6th ed. New York: Freeman and Co.
  • Lempert, R. 1989. Humility is a virtue: on the publicization of policy-relevant research. Law and Society Review 23:145-161.
  • Lewis, R.J., and S.P. Cooper 1989. Alcohol and other drugs, and fatal work-related injuries. Journal of Occupational Medicine 31(1):23-28. [PubMed: 2738746]
  • National Institute on Drug Abuse 1989. Annual Emergency Room Data: 1988. Data from the Drug Abuse Warning Network (DAWN). Rockville, Md.: National Institute on Drug Abuse.
  • National Transportation Safety Board 1990. Safety Study: Fatigue, Alcohol, Other Drugs, and Medical Factors in Fatal-to-the-Driver Heavy Truck Crashes, Vol. 1. NTSB/SS-90-01. Washington, D.C.: National Transportation Safety Board.
  • Newcomb, M.D. 1992. Substance abuse and controls in the United States: ethical and legal issues. Social Science and Medicine 35:471-479. [PubMed: 1519100]
  • Potter, W.Z. 1990. Psychotropic medications and work performance. Journal of Occupational Medicine 32(4):355-361. [PubMed: 2186167]
  • Reuter, P. 1992. Hawks ascendant: the punitive trend of American drug policy. Daedalus 121(3):15-52.
  • Rice, D.P., S. Kelman, L.S. Miller, and S. Dunmeyer 1990. The Economic Costs of Alcohol and Drug Abuse and Mental Illness: 1985. Washington, D.C.: U.S. Department of Health and Human Services.
  • Silverman, K. , S.M. Evans , E.C. Strain , and R.R. Griffiths 1992. Withdrawal syndrome after the double-blind cessation of caffeine consumption. New England Journal of Medicine 327(16):1109-1114. [PubMed: 1528206]
  • Snyder, F.R., and J.E. Henningfield 1989. Effects of nicotine administration following 12 h of tobacco deprivation: assessment on computerized performance tasks. Psychopharmacology 97:17-22. [PubMed: 2496420]
  • Snyder, F.R., F.C. Davis, and J.E. Henningfield 1989. The tobacco withdrawal syndrome: performance decrements assessed on a computerized test battery. Drug and Alcohol Dependence 23:259-266. [PubMed: 2752917]
  • Tilson, H.H. 1990. Medication monitoring in the workplace: toward improving our system of epidemiologic intelligence. Journal of Occupational Medicine 32(4):313-319. [PubMed: 2335797]
  • U.S. Public Health Service 1992. Prevention Report August/September. Washington, D.C.: U.S. Department of Health and Human Services.
  • World Health Organization 1990. International Classification of Diseases, Injuries, and Causes of Death, 10th ed. Geneva: World Health Organization.

Footnotes

1

Aside from accidents and auto fatalities, mortality associated with tobacco and alcohol abuse occur later in life than do fatalities associated with the abuse of such illicit drugs as heroin and cocaine, since deaths associated with the latter are largely attributable to the acute and immediate effects of the drug. Given that the average age of death attributed to illicit drugs is younger than that attributed to alcohol (Rice et al., 1990), illicit drug use results in higher average years of life lost (38) than alcohol (28). Nonetheless, when one takes into account the annual rate of these premature deaths (see Rice et al., 1990:135 and 136) and the relative frequency of heavy illicit drug, alcohol, and tobacco use, the social magnitude of the total premature death costs attributable to illicit drugs is still not at the level of those attributable to either alcohol or tobacco.

2

One could argue that the long-term health effects of tobacco and caffeine and their more immediate withdrawal effects on behavior also warrant attention (Silverman et al., 1992; Hughes, 1992; Snyder et al., 1989; Snyder and Henningfield, 1989).

3

Standard clinical criteria for diagnoses of use, abuse, and dependence are found in the International Classification of Diseases (ICD-10), which is the official classification system of the World Health Organization (WHO, 1990) and the Diagnostic and Statistical Manual (DSM-III-R), which is published by the American Psychiatric Association (1987). For a more thorough discussion of the idiosyncracies and commonalities across these two classification systems, the reader is referred to Babor (1992).

4

Of the 24 million tests performed annually in the United States, 18 million are performed by the approximately 100 NIDA-certified laboratories, 6 million by noncertified laboratories. Cost pet test is estimated at approximately $50 (includes laboratory fees, collection site costs, medical review officers costs, and quality control costs). Using these numbers, it is estimated that approximately $1.2 billion per year is spent on drug testing (Michael Walsh, personal communication, 1993).

Copyright 1994 by the National Academy of Sciences. All rights reserved.
Bookshelf ID: NBK236252
Contents

Views

  • PubReader
  • Print View
  • Cite this Page
  • PDF version of this title (2.6M)

In this Page

Related information

Recent Activity

ClearTurn OffTurn On

Your browsing activity is empty.

Activity recording is turned off.

Turn recording back on

See more...