Toward Environmental Justice Research, Education, and Health Policy Needs (1999) / Chapter Skim
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A Using Disease Cluster and Small-Area Analyses to Study Environmental Justice
A Using Disease Cluster and Small-Area Analyses to Study Environmental Justice
Pages 79-102
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From page 79... ...
HEALTH-EFFECTS STUDIES Barriers to Epidemiologic Studies To understand why few studies have examined the health status of minority and economically disadvantaged populations living in contaminated environments, it is helpful to define what types of health-effects studies are possible and how scientists undertake such studies. There are two principal barriers to applying traditional epidemiologic methods to issues of environmental justice: data availability and sample size.
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From page 80... ...
Sample Size In addition, many of the minority and low-income communities with environmental justice concerns are extremely small, in epidemiologic terms. Studies of small populations may not have adequate statistical power to detect a significant effect even if one exists.
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From page 81... ...
Such studies can be used for the screening of populations in regions with a high incidence of a particular disease for further study and as an aid in the design of more rigorous studies. Because the data requirements for preepidemiologic studies are far more limited than those for traditional epidemiologic methods and because the data may be more subject to underreporting or inaccurate reporting, depending on the source, the results of the analyses are less reliable.
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From page 82... ...
In addition, an exposure source is sometimes near a boundary of MCDs, so that several MCDs must be combined to capture the entire exposed population, even though only those living closest to the source are exposed, compounding the dilution problem even further. Another limitation of these data is that data on over risk factors most often are not available for individuals.
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From page 83... ...
The identification of risk factors for human irnrnunodeficiency virus transmission and AIDS also arose out of case reports from preepidemiologic studies (Centers for Disease Control, 1981~. Finally, studies of soybeans and asthma attacks have identified soybeans as a new etiologic factor for He disease, and studies of Hodgkin's disease in young adults and mesothelioma in the small village of Karain, Turkey, have helped focus further epidemiologic studies that eventually led to a reduction in the number of cases of disease (Alexander, 1992~.
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From page 84... ...
Few of the methods allow adjustment of confounding variables such as population density, ethnicity, age of residents, and so forth. Of the categories of study types investigated, space-time clusters were assessed most often, followed by spatial clusters, seasonal periodicities, temporal clusters, and occupational clusters.
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From page 85... ...
Choosing the Right Statistical Analysis Method The statistical analysis methods most often used in preepidemiologic studies are the most simple to apply: the chi-square test and the Knox test. Neither method allows adjustment for confounding factors.
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From page 86... ...
. For example, if one is concerned about a sudden increase in the incidence of asthma following the opening of a new industrial facility, one might want to look for a trend in asthma incidence over time, both before and after the opening.
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From page 87... ...
Some methods enable researchers to modify the size of the spatial interval to accommodate confounding variables such as population size (Hjalmars et al., 1996; Kulldorff, 1997; Openshaw et al., 1988; Turnbull et al., 1990~. Some methods modify the distance between geographic units to reflect the underlying population density by a statistical (Whittemore et al., 1987)
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From page 88... ...
The alternative hypothesis defines the pattern that one wishes to infer if one rejects the null hypothesis. Statistical methods are not equally powerful against all alternative hypotheses.
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From page 89... ...
is divided into a set of cells, and each is assigned an expected number of cases on the basis of the overall disease rate, possibly adjusting for population density or other risk factors. Then, the observed number of cases in each cell is compared to the expected number in each cell.
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From page 90... ...
In conducting studies to evaluate the sensitivity or statistical power of a method, the investigator sets three important parameters: the number of events, the relative risk, and the alternative hypothesis. Most often, in preepidemiologic investigations, there are a few to a few dozen cases.
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From page 91... ...
studied the statistical power of focused cluster tests using hypothetical data sets with 51, 150, and 300 people with disease. A type of trend test (the local score tests Lawson and Williams, 1993; Wailer et al., 19943)
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From page 92... ...
As demonstrated by participants at We 1989 conference on disease clusters sponsored by the Centers for Disease Control, many investigators are skeptical of the utility and statistical reliability of cluster studies (Neutra, 1990; Rothman, 1990~. Some investigators are concerned that cluster studies most often result from community awareness of high disease rates Mat .
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From page 93... ...
, in which an excess incidence of childhood leukemia appears to have persisted, even though the putative exposure, a contaminated drinking water well, was closed. (More epidemiologic research to understand this situation is under way.)
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The examples of preepidemiologic studies described above provide furler justification for their successful use win more traditional standards, although many have argued that, in epidemiology, one should not be overly concerned with p values in the context of traditional inference (Rothman, 1990b; Savitz and Olshan, 1995; Thomas et al., 1985~. RESEARCH NEEDS This summary of methods for the investigation of environmental justice issues has highlighted a number of limitations of these methods and needs for improvement.
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From page 95... ...
Finally, by compiling the results, one may begin to understand how to use these tools and what interpretations to draw from the results. Fourth, one must consider the trade-offs involved in the interpretation ot statistical significance in preepidemiologic studies.
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From page 96... ...
Such approaches have been tried in other situations in which the exposures are similar (Cardis et al., 1995; Geschwind et al., 1992; Marshall et al., 1995) , but the methods and their limitations need to be examined in the context of using preepidemiologic methods and studying issues related to environmental justice.
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From page 97... ...
This may require researchers to interview all current and former residents to get disease incidence and risk factor information. On the other hand, it may require working with local officials to break down regional data to the individual level.
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From page 98... ...
Journal of the Royal Statistical Society Series A 154(part 1~:143-155. Bithell, J
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From page 99... ...
1996. Childhood leukemia in Sweden: Using GIS and a spatial scan statistic for cluster detection.
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Journal of the Royal Statistical Society Series B 10:243-251. Moran, P
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1993. Effect of relative risk and cluster configuration on the power of the one-dimensional scan statistic.
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From page 102... ...
1981. A generalized scan statistic test for the detection of clusters.
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