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Lecture 3

AS.280.335 Lecture 3: Lecture 3 Notes

3 Pages

AS Public Health Studies, AS Earth & Planetary Sciences, EN Geography & Environmental Engineering
Course Code
Yager, James D.; Bressler, Joseph P.

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September 8, 2016 Environmental Epidemiology  Epidemiology studies the broad relationship/association between exposure and its related health outcome (disease)  Exposure-response relationship between frequency/intensity/duration of exposure and frequency of health outcome is positive (upward sloping) if adverse or negative (downward sloping) if protective o I.e. frequency of health outcome is a linear function of frequency of exposure  In order to get exposure-response association, use epidemiologic study designs o Experimental design (randomized controlled trials)  From the population, you apply exclusion criteria and derive a sample  Randomize subjects in a study to either get a treatment/exposure or not in an attempt to distribute both measurable and unmeasurable factors (i.e. age, race/ethnicity, sex, etc.) equally between the two groups  The hope is that this creates identical conditions between the two groups (unbiased way to look at the effective exposure or treatment)  However, uniformity across all factors is very challenging and randomized controlled trials are largely unethical (compromise on identical nature of groups) o Thus, mostly observational design studies o All studies measure exposure and outcomes of interest in individuals or populations  Confounder – a factor not accounted for that dilutes out a true association, whereas when you adjust for it, you see an association; however, it could also amplify an association in that when you adjust for it, the association disappears  Example: we could be introducing mercury exposure to ourselves by mercury levels in fish; however, you also consume beneficial omega-3 fatty acids o Mercury can have an adverse relationship with my myocardial infarction as a health outcome o In contrast, N-3 fatty acids could have a protective effect on myocardial infarction o If you’re not measuring both of these, the you can potentially reach an incomplete conclusion regarding the relationship between fish consumption and myocardial function o Once n-3 fatty acids as a confounding factor is adjusted for, the odds ratio goes up, meaning that there is an adverse association  1.0 is the null value for odds ratio – no effect on relationship between exposure factor and the outcome  Values in brackets are confidence intervals – lower and upper bounds  Lower bound should be above 1  Once you adjust for fatty acids (DHA), you see an effect that is more pronounced for mercury that you otherwise would not have seen o Likewise, until you adjust for mercury, you really don’t see the potential protective effect of DHA from fish consumption in the odds ratio values September 8, 2016  Odds ratio decreases below one once mercury is adjusted for indicating a protective direction o Mercury was masking an inverse association between DHA levels and the risk of myocardial infarction that only became evident after the adjustment  The ability to enroll thousands of people into a study, so that you can assure the representativeness of the population of your sample to the general population is tough. o Selection bias occurs because researchers only have enough money to get a small percentage of the general population as its sample – leads to systematic diff
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