Causal inference: The case of hygiene and health

Abstract

A fundamental goal of applied epidemiology is to determine whether a relationship between 2 factors is causal. For example, the primary purpose of an outbreak investigation is to identify what factor(s) “caused” the problem, and the purpose of the Study of the Efficacy of Nosocomial Infection Control (SENIC Project) was to measure the effect of infection control and prevention programs on rates of nosocomial infections (ie, whether strong programs were associated with or “caused” a reduction in infections). 1 Haley RW Culver DH White JW et al. The efficacy of infection surveillance and control programs in preventing nosocomial infections in US hospitals. Am J Epidemiol. 1985; 121: 182-205 Crossref PubMed Scopus (1587) Google Scholar There have been a number of historic efforts to formulate methods for valid causal inference. 2 DHHS US Surgeons General's Advisory Committee on Smoking and Health. US Department of Health, Education and Welfare, Public Health Service, Washington (DC)1964 Google Scholar , 3 Evans AS Causation and disease: the Henle-Koch postulates revisited. Yale J Biol Med. 1976; 49: 175-195 PubMed Google Scholar , 4 Hill A The environment and disease: association or causation?. Proc R Soc Med. 1965; 58: 295-300 PubMed Google Scholar , 5 Susser M Criteria of judgement. Causal thinking in the health sciences: concepts and strategies in epidemiology. in: Oxford University Press, New York1973: 140-162 Google Scholar To the layperson, association between 2 variables is often assumed to be causal, but from the epidemiologic point of view, this is not the case. For example, in studies concerning health and hygiene, associations have been found between hand hygiene interventions and a decline in diarrhea and between laundering practices and prevalence of infectious disease in the home. 6 Han AM Hlaing T Prevention of diarrhoea and dysentery by hand washing. Trans R Soc Trop Med Hyg. 1989; 83: 128-131 Abstract Full Text PDF PubMed Scopus (78) Google Scholar , 7 Shahid NS Greenough III, WB Samadi AR Huq MI Rahman N Hand washing with soap reduces diarrhoea and spread of bacterial pathogens in a Bangladesh village. J Diarrhoeal Dis Res. 1996; 14: 85-89 PubMed Google Scholar , 8 Wilson JM Chandler GN Muslihatun J Hand-washing reduces diarrhoea episodes: a study in Lombok, Indonesia. Trans R Soc Trop Med Hyg. 1991; 85: 819-821 Abstract Full Text PDF PubMed Scopus (49) Google Scholar , 9 Larson E Gomez Duarte C Home hygiene practices and infectious disease symptoms among household members. Public Health Nurs. 2001; 18: 116-127 Crossref PubMed Scopus (33) Google Scholar Such associations, however, must properly reflect an underlying causal mechanism and must have been investigated with use of rigorous methodologic procedures. 10 Buck C Popper's philosophy for epidemiologists. Int J Epidemiol. 1975; 4: 159-168 Crossref PubMed Scopus (96) Google Scholar , 11 Susser M What is a cause and how do we know one? A grammar for pragmatic epidemiology. Am J Epidemiol. 1991; 133: 635-648 PubMed Google Scholar , 12 Cook T Campbell D Quasi-experimentation: design and analysis issues for field settings. Houghton Mifflin Co, Boston1979 Google Scholar Our purpose is to discuss 1 of the historic methods devised for inferring causation and the epidemiologic concepts that relate to causal inference, with the example of health and hygiene.