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  • Title: The use of nutritional status as a second outcome measure in case-control studies of environmental risk factors for diarrhoeal diseases.
    Author: Cousens SN, Feachem RG, Daniels DL.
    Journal: Int J Epidemiol; 1989 Sep; 18(3):701-4. PubMed ID: 2807677.
    Abstract:
    Case-control studies are typically used to study the effect of several factors on the risk/incidence rate of a single disease. This paper describes a particular situation in which it is of interest to study the effect of a factor--improved sanitation facilities--on the risk/incidence rate of two 'diseases'--the incidence rate of diarrhoea and the risk of undernutrition. The conditions under which it is valid to perform an analysis of the association between the risk factor (unimproved sanitation) and a second outcome variable (undernutrition) are examined. If the effect of exposure status (improved/unimproved sanitation facilities) on the propensity to report an episode of diarrhoea is independent of the effect of nutritional status it appears that such an analysis may be valid. There must also be no interaction between the risk factor (unimproved sanitation) and the second outcome (undernutrition) with respect to their effects as risk factors for the first outcome variable (diarrhoea incidence rate). Several water supply and sanitation experts proposed using the case control method to learn of health benefits of water supply and sanitation projects. They believed it to have 3 advantages over the traditional longitudinal studies: smaller sample size required, chance of misclassifying a disease is minimized, and any evaluation may begin after the intervention is known to be working. 3 epidemiologists designed a theoretical model incorporating 2 outcomes--diarrhea and nutritional status to be used in a study to evaluate the impact of the rural sanitation program in Lesotho. Each child is placed in 1 of the following categories: exposed, unexposed, undernourished, and adequately nourished. The model's 1st condition for an unbiased odds ratio requires that the effect of a child's exposure status on his/her likelihood to report for diarrhea is independent of any effect of nutritional status. If exposure is associated with the distance from home to the clinic, the likelihood of a child coming to the clinic for a diarrhea episode may be related to the exposure status. If so, recording distance allows the effect of a child's exposure status to be controlled during the analysis. Further, the model assumes that the population is in a state of equilibrium nutritionally. For example, undernourished children are balanced by an equal number of children whose nutritional status is improving and 1 diarrhea episode for a recruited child does not change that child's nutritional status. Therefore, if nutritional status does not change the association between exposure to water supply/sanitation facilities and diarrhea, then this 2nd condition will maintain an unbiased estimate of the prevalence odds ratio. If these conditions are met, this analysis should be valid.
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