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Objectives
• To review age, cohort, and period effect
• To discuss study designs
• To discuss measures of disease occurrence
• To practice interpretation of measures of disease occurrence
Study Designs
Descriptive Analytic Experimental
correlational
case report/case series
cross-sectional
case control
cohort
clinical trial
community trial
Study Design
• Descriptive– To describe the population at risk– To develop hypotheses
• Analytic– To test hypotheses
• Experimental– To test hypotheses
Epidemiologic Assumptions
• Human disease does not occur at random.
• Human disease has causal and preventive factors that can be identified through systematic investigation of populations.
Ecological Studies
• Units of analysis are usually geographically defined populations
• example: the correlation between alcohol outlets and violent crimes)
• Example: the association between “broken windows” (a measure of social decay) and sexually transmitted disease rates)
Examples• Aggregate measure – summarizes characteristics
of individuals within a group. (eg. mean income, percent male, education level from census track data).
• Environmental measure – physical characteristics of the geographic (e.g. location percentage of homes that are blighted, percentage of of schools without after-school programs).
• Global measure – characteristics that are not reducible to the individual level Aggregate –Environmental – (e.g. drinking age laws, social security benefits)
Difficulties with interpretation of ecological studies
• No temporality• Can’t really adjust for confounders because
the analysis is on the aggregate level• Susceptible to ecological fallacy or aggregate
bias – when the association observed on the aggregate level does not necessarily represent the association that exists at the individual level.
Cohort Study
• A defined population is identified, persons are categorized by exposure then followed to an outcome of interest.
• There is a comparison of the rate of the outcome among those who are exposed compared to those who are not exposed. (measure of association – relative risk)
• Persons who are lost to follow-up are called censored or withdrawls
Cohort study (con’t)
• Can be concurrent (prospective), non-concurrent (retrospective or historical) or mixed
Table 1. Factors associated with an incident STD among HIV-infected women in Cox Regression Analysis (N=741)
n
Incident STD (n=109)
Hazard Ratio (95% C.I.)
Race Non-white
White
624 117
16.0% 7.7%
1.41 ( .70, 2.85)
Age < 22 years 22 years
141 600
27.0% 11.8%
1.92 (1.25, 2.96)*
CD4
200/mm3 < 200/mm3
506 161
18.6% 8.7%
1.03 ( .58, 1.83)
Substance use Yes No
284 457
18.7% 12.3%
1.71 (1.13, 2.59)*
History of STD Yes No
145 596
49.7% 6.2%
9.80 (6.44, 14.9 )*