7.1 INTRODUCTION7.2 BASIC ELEMENTS OF EPIDEMIOLOGICAL STUDIES7.2.1 Formulation of the study question or hypothesis7.2.2 Selection of study populations7.2.4 Measurements of exposure and disease status7.2.5 Analysis of the relationship between exposure and disease7.2.6 Evaluation of the role of chance7.2.7 Evaluation of the role of bias7.3 TYPES OF STUDY7.3.1 Descriptive studies7.3.2 Analytical studies7.3.2.1Ecological (or correlational) studies7.3.2.2Cross-sectional studies7.3.2.3Cohort studies7.3.2.4Case-control studies7.3.3 Experimental or intervention studies7.4 USES OF EPIDEMIOLOGY IN THE SETTING OR EVALUATING OF MICROBIOLOGICAL GUIDELINES7.5 CASE STUDIES7.5.1 Recreational water case study7.5.2 Wastewater reuse case study7.3.3 Drinking-water case study7.4 DISCUSSION7.5 IMPLICATIONS FOR INTERNATIONAL GUIDELINES AND NATIONAL REGULATIONS7.6 REFERENCES© 2001 World Health Organization (WHO). Water Quality: Guidelines, Standards and Health. Edited byLorna Fewtrell and Jamie Bartram. Published by IWA Publishing, London, UK. ISBN: 1 900222 28 07Epidemiology: a tool for theassessment of riskUrsula J. Blumenthal, Jay M. Fleisher,Steve A. Esrey and Anne PeaseyThe purpose of this chapter is to introduce and demonstrate the use of a key toolfor the assessment of risk. The word epidemiology is derived from Greek and itsliteral interpretation is ‘studies upon people’. A more usual definition, however,is the scientific study of disease patterns among populations in time and space.This chapter introduces some of the techniques used in epidemiological studiesand illustrates their uses in the evaluation or setting of microbiologicalguidelines for recreational water, wastewater reuse and drinking water.7.1 INTRODUCTIONModern epidemiological techniques developed largely as a result of outbreakinvestigations of infectious disease during the nineteenth century.Water quality - Guidelines, standards and health: Assessment of risk and risk management for water-related infectious disease Edited by Lorna Fewtrell and Jamie Bartram, World Health Organization, Geneva http://www.who.int/water_sanitation_health/dwq/iwachap7.pdf136 Water Quality: Guidelines, Standards and HealthEnvironmental epidemiology, however, has a long history dating back to Romanand Greek times when early physicians perceived links between certainenvironmental features and ill health.John Snow’s study of cholera in London and its relationship to water supply(Snow 1855) is widely considered to be the first epidemiological study (Baker etal. 1999). Mapping cases of cholera, Snow was able to establish that cases ofillness were clustered in the streets close to the Broad Street pump, withcomparatively few cases occurring in the vicinity of other local pumps.Epidemiological investigations can provide strong evidence linking exposureto the incidence of infection or disease in a population. They can provideestimates of the magnitude of risk related to a particular level of exposure ordose and so can be used in the evaluation of appropriate microbiological qualityguideline levels or standards. Epidemiological methods can quantify theprobability that observed relationships occurred by chance factors and they alsohave the potential to control for other risk factors and/or confounders of theoutcome illness being studied. Epidemiological studies used for the evaluationor setting of guidelines must be of high quality, so that there is confidence in thevalidity of the results.The following sections outline the basic elements of epidemiological studies(including comments on features that are important for high quality studies), thedifferent types of epidemiological study, and the use of epidemiology inguideline setting, with case studies of the use of epidemiology in recreationalwater, drinking water and wastewater reuse settings.7.2 BASIC ELEMENTS OF EPIDEMIOLOGICALSTUDIESThe basic elements of an epidemiological study can be characterised as follows:• formulation of the study question or hypothesis• selection of study populations and study samples• selection of indicators of exposure• measurement of exposure and disease• analysis of the relationship between exposure and disease• evaluation of the role of bias• evaluation of the role of chance.These elements will be considered here in a simplified format. Readers arereferred to epidemiology textbooks for consideration of the factors in moredetail (Beaglehole et al. 1993; Friis and Sellers 1996; Hennekens and BuringEpidemiology: a tool for the assessment of risk 1371987; Rothman and Greenland 1998). The case studies include examples of theelements described here.7.2.1 Formulation of the study question or hypothesisThe study question must be formulated so that it can be tested using statisticalmethods. For example:• Exposure to wastewater (meeting the WHO guidelines) comparedwith no exposure to wastewater does not increase the rate ofAscaris infection.The null hypothesis (which implies there is no relationship betweenpostulated cause and effect) states that observed differences are due to samplingerrors (i.e. to chance). Stated in the null form, the propositions are refutable andcan be assessed using statistical tests (see section 7.2.6).7.2.2 Selection of study populationsA study population exposed (to the factor of interest) and a control population(not exposed to the factor of interest) need to be selected (except in aprospective cohort study where a single cohort is studied and analysis is onexposure status). A sample from the exposed and control populations needs tobe selected to be as similar as possible in all factors other than the factor ofinterest e.g. socio-economic status, and other risk factors for the diseaseoutcome of interest. Since samples are never totally similar, we need to recordpossible confounding factors and control for them in the analysis (see below).For enteric infections arising from exposure to contaminated water, such factorswould include sanitation, personal hygiene, drinking-water supply, foodhygiene, and travel. It is important that both exposure and disease can bemeasured as accurately as possible in the chosen populations. For example, instudies on drinking water, the drinking water source (and therefore the quality)for each household needs to be known accurately. In most studies, a sample willbe selected from a larger population exposed to the factor of interest, using asampling frame. This needs to be done so that it is
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