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Institute of Medicine (US) Forum on Microbial Threats; Knobler SL, O'Connor S, Lemon SM, et al., editors. The Infectious Etiology of Chronic Diseases: Defining the Relationship, Enhancing the Research, and Mitigating the Effects: Workshop Summary. Washington (DC): National Academies Press (US); 2004.

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The Infectious Etiology of Chronic Diseases: Defining the Relationship, Enhancing the Research, and Mitigating the Effects: Workshop Summary.

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, M.D., , M.D., Ph.D., , M.S., , M.S., and , Ph.D.

Center for Global Health, University of Virginia School of Medicine; and Federal University of Ceara, Fortaleza, Brazil

The assessment of the global burden of diseases is increasingly important in recognizing and analyzing their importance as well as the priority of economic investments in their amelioration. In this perspective we recognize the quality of life or years lived with varying degrees of disability in addition to the quantity of life lost to premature mortality, as important outcomes or consequences of all diseases or conditions. Recognizing disability or quality of life is especially important, as mortality from a growing list of acute diseases is reduced, and chronic diseases or long-term consequences of diseases or conditions are now being appreciated. Only such a global view can begin to capture the full human and economic costs of diseases, injuries, or other conditions. Only as these true costs are appreciated, can we affect the necessary investments in their alleviation (Guerrant, 2001; Guerrant and Blackwood, 1999).

Importance of Measuring Morbidity as well as Mortality

Major advances have been made in understanding the quality and quantity elements of health outcomes and the global burdens of disease. Two of these “quality of life” measures are Quality-Adjusted Life Years (QALYs) and Disability-Adjusted Life Years (DALYs). QALYs have been devised by economists to capture both quality and quantity elements of a health care outcome in a single measure, and have been used primarily in assessing the effectiveness of specific interventions to improve health. However, QALYs suffer problems of subjective value assignments that vary considerably with who makes the choices, and they do not capture wider benefits (externalities) that may accrue to society, family, or friends.

DALYs involve not only calculating age-specific mortality (as years of potential life lost [YPLL] to fatal conditions) but also taking into account the quality of life affected by disabilities (by formulating years lost to disability [YLD] with nonfatal conditions, injuries, and diseases) (Murray et al., 1994; Murray and Lopez, 1997). In calculating DALYs, perfect health is weighted as 0 disability with disability weights progressing to 1, the equivalent of death. DALYs have the advantages that they can also help assess effectiveness of interventions as well as the burden of disease and are standardized to permit age weighting and comparability across studies.

All conditions affecting health as well as interventions that prevent or reverse the adverse effects of these conditions are measured in economic as well as human terms. These include, in addition to the causes of death and the YPLL due to premature mortality, the morbidity costs or YLD from conditions that impair the ability of individuals to reach their full human and economic potential or productivity. As causes of premature mortality are brought under control worldwide, the morbidity costs are becoming increasingly recognized and their quantitation is increasingly important. Thus, in addition to diseases or conditions like meningitis, AIDS, or automobile accidents that are often fatal at young ages and are thus responsible for disproportionately greater years of life lost, we must also weigh the burden of chronic diseases, like arthritis or depression, that often disable much more than they kill. Both YPLL and YLD are included in the DALYs that are being used to assess the burdens of all diseases or conditions that threaten healthy life worldwide, as well as the “cost-effectiveness” of interventions designed for their amelioration. Both mortality (YPLL) and morbidity (YLD) pose profound economic costs, whether a young, productive working parent dies with AIDS or violence, or whether a child with repeated bouts of diarrhea, parasitic infection, or malnutrition fails to develop normally to meet his or her full human and economic potential.

It is just such an analysis that has brought appropriate attention to conditions like neuropsychiatric diseases or depression that kill few but disable many. Likewise, from placebo-controlled prospective studies of albendazole treatment of helminthic infections in Kenyan and Jamaican schoolchildren, intestinal helminths have been found to impair growth, fitness, and even cognitive function (Adams et al., 1994; Nokes et al., 1992a,b; Nokes and Bundy, 1992; Stephenson et al., 1993). Such studies have enabled Chan and Bundy to suggest potential recalculation of the long-term impact of childhood helminthic infections on DALYs to essentially double their previous values (Chan et al., 1994; Guerrant and Blackwood, 1999).

Indeed, the disability component of the DALY calculations for malnutrition and the “tropical cluster” (trypanosomiasis, Chagas' disease, schistosomiasis, and leishmaniasis), like neuropsychiatric conditions, chronic obstructive lung disease, and rheumatoid arthritis, outweigh their mortality components (Guerrant and Blackwood, 1999; Murray and Lopez, 1997). However, the initially calculated DALY for diarrheal diseases, from a 1997 assessment (Murray and Lopez, 1997), initially comprised 95 percent mortality (YPLL) and only 5 percent disability (YLD, from the transient 10 percent incapacitation during just the overt diarrheal illness [i.e., liquid stools] itself). No long-term disability from repeated dehydrating and malnourishing diarrheal illnesses in the critical formative developmental first 2 years of life is considered, largely because there had been no data to suggest such long-term effects (Guerrant and Blackwood, 1999).

Potential Long-Term Morbidity from Diarrheal Disease

The challenge is to obtain data implicating specific diseases or conditions with long-term impaired outcomes. Best studied perhaps are nutritional effects that may even involve genetic “imprinting” from the regulation of critical developmental genes at pivotal times by DNA methylation, that might further extend the developmental impact of early childhood illnesses perhaps even beyond 2–3 generations (Golden, 1994). In addition, iron deficiency has a well recognized impact on cognitive development (Basta et al., 1979; Soewondo et al., 1989). Nevertheless, despite the lack of a specific single drug (like albendazole for intestinal helminths) to control diarrheal diseases, long-term cohort studies are now enabling associations to be made of heavy early childhood disease burdens with later functional as well as nutritional outcomes. The growing evidence for lasting disability consequences of early childhood diarrhea and specific parasitic infections (including cryptosporidiosis and intestinal helminthic infections in the first 6–24 months of life) is presented in Table 2-1.

TABLE 2-1. Evidence for Lasting Disability Effects from Early Childhood Diarrhea.


Evidence for Lasting Disability Effects from Early Childhood Diarrhea.

Perhaps one of the greatest of all overlooked costs of the diseases of poverty, such as diarrhea and intestinal parasitic infections, are the increasingly recognized, long-term developmental impact of early childhood illnesses, so common in developing areas. For example, we are now learning that the 4–8 dehydrating, malnourishing diarrheal illnesses that often occur each year in the critically formative first two years of life may have profound, lasting consequences for impaired fitness, growth, cognitive development, and school performance several years later. Initial studies in Northeast Brazil show reduced fitness 4 to 6 years later associated with early childhood diarrhea, and specifically with cryptosporidial infections in the first 2 years of life, independent of respiratory illnesses, anthropometry, anemia, and intestinal helminths (Guerrant and Blackwood, 1999). The fitness deficits alone that associate with the median diarrhea burdens in the first 2 years of life in these studies in Northeast Brazil are comparable to that associated with a 17 percent decrement in work productivity in Zimbabwe sugarcane workers (Guerrant et al., 1999; Ndamba et al., 1993).

Furthermore, these early childhood diarrheal illnesses and intestinal helminthic infections in the first 2 years of life independently and additively associate with substantial long-term linear growth shortfalls that continue beyond six years of age (totaling an average of 8.2 cm [3 ¼ inches] growth shortfall at 7 years old, 3.6 cm with diarrhea alone after controlling for early childhood intestinal helminthic infections) (Moore et al., 2001). In addition, longitudinal studies in Peru (Checkley et al., 1997, 1998) have also shown that cryptosporidial infections (even without overt diarrhea) in young or stunted children predispose to an average 1 cm growth shortfall 1 year after infection.

We also find significant associations of early childhood diarrhea with long-term cognitive deficits (by standard “Test of Nonverbal Intelligence” [TONI]) even when controlling for maternal education, breast feeding duration, and early helminthic infections (Niehaus et al., 2002). Furthermore, WISC (Wechsler Intelligence Scale for Children; The Psychological Corp, San Antonio, TX) coding and digit span scores were lower in children with persistent diarrheal illnesses in their first 2 years of life, even when controlling for helminths and maternal education (Niehaus et al., 2002). And these effects are seen in a “best case” scenario in which we have documented substantial improvements in disease rates and in nutritional status over the several years in which we have conducted close, long-term surveillance of this population (Moore et al., 2000), effects that we have subsequently not found in other nearby shantytown communities that had not been under such intensive surveillance (Lima, Guerrant et al., unpublished observations).

We are now finding that these correlations of early childhood diarrhea are also extending to school performance, with significant associations of diarrhea in the first 2 years of life with delayed age at starting school and age for grade that remain even after controlling for maternal education and (also affected) stature. Late starters are also two-fold more likely to have experienced cryptosporidial infections (Lorntz et al., 2000).

A recent report describes the significant associations of stunting in the first 2 years of life and multiple episodes of Giardia infection with impaired intelligence quotients on the WISC-R test among children in Peru (Berkman et al., 2002). This is the setting in which diarrhea is also associated with reduced WISC-R scores albeit not independently of its association with stunting. This is also the setting in which cryptosporidial infections are associated with persistent stunting as well (Checkley et al., 1997, 1998).

Most recently we have launched studies of sensitive measures of higher order frontal lobe development and critical “executive” functioning that predict functional recovery from brain injury in children. We conducted semantic and phonetic fluency testing among 74 children who have now reached 6–12 years old from our prospective surveillance population (with their diarrheal illnesses recorded from birth). Early childhood diarrhea, whether measured by total numbers of episodes or as days of diarrhea in the first 2 years of life was a highly significant predictor of total fluency scores at 6–12 years of age (i.e., 4–10 years later). Impressively, early childhood diarrhea remained a significant predictor of fluency even when controlling for maternal education and for household income (p = 0.02; beta = −0.31)1 or when controlling for birth size (p = 0.007; beta = −0.325) or height-for-age Z score (HAZ) at 6.5 years old. Since early childhood diarrhea has such profound effects on TONI III scores and on HAZ at age 2 years old, its association with fluency was not significantly independent of TONI III or HAZ at 2 years old. The persistence of strong associations of early diarrhea with fluency to 6–12 years old and its independence of HAZ at birth and at 6.5 years old (despite persistent associations of diarrhea with HAZ to 6–7 years old) suggests that despite the growth effects recovering in part, the lasting impact of early childhood diarrhea does not recover and is even greater on functional verbal fluency than on growth. We conclude that the higher frontal lobe executive functioning impairment seen at 6–12 years old associated with diarrhea in the first 2 years of life, especially with impaired schooling, growth and cognition, suggest that early childhood diarrhea results in critical neurodevelopmental impairment that greatly magnifies the importance of ameliorating these diarrheal illnesses and their long-term consequences.

These potential consequences of early childhood malnourishing and dehydrating diarrheal illnesses should not be a great surprise when one considers the importance of early childhood years in human brain development (Dobbing, 1985; 1990; Dobbing and Sands, 1985; Niehaus et al., 2002). Unlike other species such as monkeys, sheep or opossums, which have most of their brain development in utero, it is during the first 2 years of life in humans that the major brain growth and synapse formation occurs. Furthermore, if impaired at this formative stage, it is apparently difficult if not impossible to compensate or build these synapses later in life. Add to this the recognized potential for genetic imprinting noted above, and the duration of impact of early childhood illnesses may well be lifelong and even extend even to the next generation(s).

Thus the disability impact and ultimate societal costs of these early childhood diarrheal illnesses of poverty is potentially far greater and more critical a global investment than is generally appreciated, i.e., a global “tax” that is paid for the impaired work productivity in the global economy because these largely preventable illnesses continue unabated. Thus, beyond their obvious human toll, the diseases of poverty may well require an economic investment (as they are readily prevented) that we cannot afford not to make.

Persistent High Diarrhea Morbidity Despite Improving Mortality

The importance of an accurate assessment of the YLD, years lost to disability from early childhood illnesses like diarrheal diseases is further accentuated by the striking relative shift from mortality to morbidity seen over recent decades. Despite clear reductions in diarrhea mortality (from 4.2 to 3.3 to 2.5 million) from 1955 to the present (Bern et al., 1992; Kosek et al., 2003; Snyder and Merson, 1982), the morbidity rates from a third 10-year update review (Kosek et al., 2003) have not decreased; instead, with the fastest growing populations occurring in the poorest areas with the highest disease rates, the total global morbidity from diarrhea has actually substantially increased. The potential impact of these still common early childhood diarrheal illnesses on long-term development or disability only further adds to their morbidity costs.

Refining DALYs for Diarrheal Disease

As shown in the first row of Table 2-2, following the standard formulas with age-weighting and discounting at 3 percent, and all disability falling into the lowest class (weight of 0.096), the DALY calculations for diarrheal diseases are presented.

TABLE 2-2. Revised Calculations of Disability-Adjusted Life Years (DALYs) for Diarrheal Diseases.


Revised Calculations of Disability-Adjusted Life Years (DALYs) for Diarrheal Diseases.

The morbidity in 0–4 year olds is presented in 5 different scenarios as follows:

  • Scenario 1 applies the original assumptions by Murray and Lopez of 2.27 million attacks of 1 week duration, in which the 1.3 million DALYs from morbidity in 0–4 year olds represents 1 percent of the total of 100.9 million global diarrhea DALYs.
  • Scenario 2 assumes that 17 percent of 0–4 year olds (or 33 percent at half the 9.6 percent disability weight) are at risk of at least 1 diarrheal attack (or a diarrhea burden) which could have life-long disability (with a life expectancy of 81.25 years as used by Murray and Lopez).
  • Scenario 3 assumes that 25 percent of 0–4 year olds (or 50 percent at half the 9.6 percent disability weight) are at life-long risk.
  • Scenario 4 assumes that 10 percent of 0–4 year olds (or 50 percent at 20 percent of the 9.6 percent disability weight, i.e., half experience a 2 percent lifelong disability).
  • Scenario 5 assumes that only 5 percent of 0–4 year olds (or half experience a 1 percent life-long disability).

Thus, a 1 to 4.8 percent disability affecting one-third to one-half of 0–4-year-old children would increase the total global diarrhea DALYs to 2 to 6-fold the current estimates. Considered differently, for every 5 percent of children affected lifelong, DALYs increase by about 100 million; 25 percent of children affected would increase current DALY estimates by over six-fold; only 5 percent affected lifelong (or 10 percent affected for only 25 years) would more than double the total global diarrhea DALYs (Guerrant et al., 2002a).

Add to this the concept that even subclinical enteric infections that may alter critical absorptive function without necessarily producing overt symptoms of liquid stools, like those with Cryptosporidium or enteroaggregative E. coli may impair growth (Checkley et al., 1997, 1998; Steiner et al., 1998), or impede the absorption of (and potentially thus enhance resistance to) key anti-HIV or anti-tuberculosis drugs (Lima et al., 1997; Brantley et al., 2003), and the potential cost of these diseases of poverty, inadequate water, and inadequate sanitation become increasingly unacceptable.


Critical to understanding and making this case for investing adequate resources in the presentation or amelioration of the diseases of poverty like diarrhea is obtaining solid information about the potential long-term correlates with illness rates and even subclinical infections, controlling to the extent possible the numerous confounding variables, and careful studies of potential interventions that could alter these adverse outcomes. Only improved data and careful, accurate analyses will direct adequate attention to alleviation of these diseases of poverty that are so potentially costly to human and societal development for us all.


  • Adams EJ, Stephenson LS, Latham MC, Kinoti SN. Physical activity and growth of Kenyan school children with hookworm, Trichuris trichiura and Ascaris lumbricoides infections are improved after treatment with albendazole. Journal of Nutrition. 1994;124:1199–1206. [PubMed: 8074755]
  • Agnew DG, Lima AA, Newman RD, Wuhib T, Moore RD, Guerrant RL, Sears CL. Cryptosporidiosis in northeastern Brazilian children: association with increased diarrhea morbidity. Journal of Infectious Diseases. 1998;177:754–760. [PubMed: 9498458]
  • Basta SS, Soerkirman, Karyadi D, Scrimshaw NS. Iron deficiency anaemia and the productivity of males in Indonesia. The American Journal of Clinical Nutrition. 1979;32:916–925. [PubMed: 107787]
  • Berkman DS, Lescano AG, Gilman RH, Lopez SL, Black MM. Effects of stunting, diarrhoeal disease, and parasitic infection during infancy on cognition in late childhood: a follow-up study. Lancet. 2002;359:564–571. [PubMed: 11867110]
  • Bern C, Martines J, de Zoysa I, Glass RI. The magnitude of the global problem of diarrhoeal disease: a ten-year update. Bulletin of the World Health Organization. 1992;70:705–714. [PMC free article: PMC2393403] [PubMed: 1486666]
  • Brantley RK, Williams KR, Silva TM, Sistrom M, Thielman NM, Ward H, Lima AA, Guerrant RL. AIDS-associated diarrhea and wasting in Northeast Brazil is associated with subtherapeutic plasma levels of antiretroviral medications and with both bovine and human subtypes of Cryptosporidium parvum. Brazilian Journal of Infectious Diseases. 2003;7:16–22. [PubMed: 12807688]
  • Chan MS, Medley GF, Jamison D, Bundy DA. The evaluation of potential global morbidity attributable to intestinal nematode infections. Parasitology. 1994;109:373–387. [PubMed: 7970892]
  • Checkley W, Gilman RH, Epstein LD, Suarez M, Diaz JF, Cabrera L, Black RE, Sterling CR. Asymptomatic and symptomatic cryptosporidiosis: their acute effect on weight gain in Peruvian children. American Journal of Epidemiology. 1997;145:156–163. [PubMed: 9006312]
  • Checkley W, Epstein LD, Gilman RH, Black RE, Cabrera L, Sterling CR. Effects of Cryptosporidium parvum infection in Peruvian children: growth faltering and subsequent catchup growth. American Journal of Epidemiology. 1998;148:497–506. [PubMed: 9737562]
  • Dobbing J. Infant nutrition and later achievement. The American Journal of Clinical Nutrition. 1985;41:477–484. [PubMed: 3881926]
  • Dobbing J. Boyd Orr memorial lecture. Early nutrition and later achievement. Proceedings of the Nutrition Society. 1990;49:103–118. [PubMed: 2236081]
  • Dobbing J, Sands J. Cell size and cell number in tissue growth and development. An old hypothesis reconsidered. Archives Francaises de Pediatrie. 1985;42:199–203. [PubMed: 4004482]
  • Golden MH. Is complete catch-up possible for stunted malnourished children? European Journal of Clinical Nutrition. 1994;48:S58–S70. [PubMed: 8005092]
  • Guerrant DI, Moore SR, Lima AA, Patrick PD, Schorling JB, Guerrant RL. Association of early childhood diarrhea and cryptosporidiosis with impaired physical fitness and cognitive function four-seven years later in a poor urban community in northeast Brazil. The American Journal of Tropical Medicine and Hygiene. 1999;61:707–713. [PubMed: 10586898]
  • Guerrant RL. The unacceptable costs of the diseases of poverty. Current Infectious Disease Reports. 2001;3:1–3. [PubMed: 11177724]
  • Guerrant RL, Blackwood BL. Threats to global health and survival: the growing crises of tropical infectious diseases—our “unfinished agenda” Clinical Infectious Diseases. 1999;28:966–986. [PubMed: 10452620]
  • Guerrant RL, Kosek M, Lima AA, Lorntz B, Guyatt HL. Updating the DALYs for diarrhoeal disease. Trends in Parasitology. 2002;18:191–193. [PubMed: 11983588]
  • Guerrant RL, Kosek M, Moore S, Lorntz B, Brantley R, Lima AA. Magnitude and impact of diarrheal diseases. Archives of Medical Research. 2002;33:351–355. [PubMed: 12234524]
  • Kosek M, Bern C, Guerrant RL. The global burden of diarrhoeal disease, as estimated from studies published between 1992 and 2000. Bulletin of the World Health Organization. 2003;81:197–204. [PMC free article: PMC2572419] [PubMed: 12764516]
  • Lima AA, Silva TM, Gifoni AM, Barrett LJ, McAuliffe IT, Bao Y, Fox JW, Fedorko DP, Guerrant RL. Mucosal injury and disruption of intestinal barrier function in HIV-infected individuals with and without diarrhea and cryptosporidiosis in northeast Brazil. American Journal of Gastroenterology. 1997;92:1861–1866. [PubMed: 9382053]
  • Lima AA, Moore SR, Barboza MS, Soares AM, Schleupner MA, Newman RD, Sears CL, Nataro JP, Fedorko DP, Wuhib T, Schorling JB, Guerrant RL. Persistent diarrhea signals a critical period of increased diarrhea burdens and nutritional shortfalls: A prospective cohort study among children in northeastern Brazil. Journal of Infectious Diseases. 2000;181:1643–1651. [PubMed: 10823764]
  • Lorntz, et al. Presentation at the ASTMH.2000.
  • Moore SR, Lima AA, Schorling JB, Barboza MS Jr, Soares AM, Guerrant RL. Changes over time in the epidemiology of diarrhea and malnutrition among children in an urban Brazilian shantytown, 1989 to 1996. International Journal of Infectious Diseases. 2000;4:179–186. [PubMed: 11231179]
  • Moore SR, Lima AA, Conaway MR, Schorling JB, Soares AM, Guerrant RL. Early childhood diarrhoea and helminthiases associate with long-term linear growth faltering. International Journal of Epidemiology. 2001;30:1457–1464. [PubMed: 11821364]
  • Murray CJ, Lopez AD, editors. The Global Burden of Disease: A Comprehensive Assessment of Mortality and Disability from Diseases, Injuries, and Risk Factors in 1900 and Projected to 2020. Cambridge, MA: Harvard University Press; 1997.
  • Murray CJ, Lopez AD, Jamison DT. The global burden of disease in 1990: summary results, sensitivity analysis and future directions. Bulletin of the World Health Organization. 1994;72:495–509. [PMC free article: PMC2486716] [PubMed: 8062404]
  • Ndamba J, Makaza N, Munjoma M, Gomo E, Kaondera KC. The physical fitness and work performance of agricultural workers infected with Schistosoma mansoni in Zimbabwe. Annals of Tropical Medicine & Parasitology. 1993;87:553–561. [PubMed: 8122916]
  • Newman RD, Sears CL, Moore SR, Nataro JP, Wuhib T, Agnew DA, Guerrant RL, Lima AA. Longitudinal study of Cryptosporidium infection in children in northeastern Brazil. Journal of Infectious Diseases. 1999;180:167–175. [PubMed: 10353875]
  • Niehaus MD, Moore SR, Patrick PD, Derr LL, Lorntz B, Lima AA, Guerrant RL. Early childhood diarrhea is associated with diminished cognitive function 4 to 7 years later in children in a northeast Brazilian shantytown. The American Journal of Tropical Medicine and Hygiene. 2002;66:590–593. [PubMed: 12201596]
  • Nokes C, Bundy DA. Trichuris trichiura infection and mental development in children. Lancet. 1992;339:500. [PubMed: 1346859]
  • Nokes C, Grantham-McGregor SM, Sawyer AW, Cooper ES, Bundy DA. Parasitic helminth infection and cognitive function in school children. (Series B Biological Sciences).Proceedings of the Royal Society of London. 1992;247:77–81. [PubMed: 1349184]
  • Nokes C, Grantham-McGregor SM, Sawyer AW, Cooper ES, Robinson BA, Bundy DA. Moderate to heavy infections of Trichuris trichiura affect cognitive function in Jamaican school children. Parasitology. 1992;104:539–547. [PubMed: 1641252]
  • Snyder JD, Merson MH. The magnitude of the global problem of acute diarrhoeal disease: a review of active surveillance data. Bulletin of the World Health Organization. 1982;60:605–613. [PMC free article: PMC2536091] [PubMed: 6982783]
  • Soewondo S, Husaini M, Pollitt E. Effects of iron deficiency on attention and learning processes in preschool children: Bandung, Indonesia. The American Journal of Clinical Nutrition. 1989;50:667–673. [PubMed: 2773844]
  • Steiner TS, Lima AA, Nataro JP, Guerrant RL. Enteroaggregative Escherichia coli produce intestinal inflammation and growth impairment and cause interleukin-8 release from intestinal epithelial cells. Journal of Infectious Diseases. 1998;177:88–96. [PubMed: 9419174]
  • Stephenson LS, Latham MC, Adams EJ, Kinoti SN, Pertet A. Physical fitness, growth and appetite of Kenyan school boys with hookworm, Trichuris trichiura and Ascaris lumbricoides infections are improved four months after a single dose of albendazole. Journal of Nutrition. 1993;123:1036–1046. [PubMed: 8505663]



Parts of this paper have been published in a perspective article (Guerrant et al, 2002a) and in a review (Guerrant et al., 2002b).


The statistical symbol p stands for the probability that the observed difference could have been obtained by chance alone, given random variation and a single test of the null hypothesis.

Copyright © 2004, National Academy of Sciences.
Bookshelf ID: NBK83698


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