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Unformatted text preview: Cohort Studies Part 2 Cohort Studies – Part 2 Dr Borenstein Dr. Borenstein NO AUDIO ON THIS SLIDE. CLICK NEXT TO CONTINUE >> Back to Cohorts ¡ People accrue time in the study. We need to document each person’s time accurately. ¢ Always use time unit when expressing either CI or ID rates ¢ Always use time unit when expressing either CI or ID rates. ¢ ID rate is a true “rate” – implies “speed” of developing disease. ¢ CI is a rate because we use a time unit (e g 10 per 1 000 ¢ CI is a rate because we use a time unit (e.g., 10 per 1,000 people per year) but does not imply “speed” the way ID does. ¢ Prevalence “rate” is not really a rate just the proportion of ¢ Prevalence rate is not really a rate, just the proportion of people who have the disease in a population at a given (specified) time. ¢ We can estimate person time (N x ) but this number ¢ We can estimate person-time (N x t i ) but this number (denominator) will always be larger than actual person- time: incidence density is (a) more accurate and (b) will reflect a rate that is a bit larger than a rate where T is assumed to be complete. ¡ Ex: 10/1000 py= 0.01 (CI=Nxt i ) 10/867=0.015 (ID) Cohort studies ¡ Selection of the comparison (unexposed) group ¡ Selection of the comparison (unexposed) group ¢ The unexposed group should be similar to the exposed group except for the exposure ¡ In RCT, Rx and non-Rx groups will be exactly similar if (a) N is large enough and (b) randomization is accomplished ¡ Can match exposed to unexposed on important characteristics on which they might differ and which are related to risk of disease, but very difficult to do and not commonly done ¡ If unexposed group comes from same base population as exposed group, the two groups will population as exposed group, the two groups will be similar in that they arose from the same population Design issues in cohort studies: Design issues in cohort studies: sources of exposure data ¡ Pre-existing records (medical records, employment records) ¢ Advantages: ¡ Cheap ¢ Disadvantages ¡ May be inaccurate ¡ Relatively easy to work with ¡ Usually unbiased ¡ May have missing data Ma not contain (exposures documented before outcomes) ¡ May not contain data needed, esp. re: potential ¡ Good info on meds, procedures, etc. confounders/effect- modifiers Design issues in cohort studies: sources of exposure data, cont’d ¡ Self-report (in-person, telephone, mail) ¢ Advantages: ¡ Can design ¢ Disadvantages ¡ Subject to response sophisticated, structured questionnaires and (recall) bias (but not selectively by case status) S bj t t question S on as many factors as necessary ¡ Subject to interviewer/observer bias (but not selectively by case status necessary by case status) ¡ Expensive Design issues in cohort studies: sources of exposure data, cont’d ¡ Physical examinations, psychological testing, biologic specimen collection, laboratory testing, environmental monitoring of home/person/ workplace...
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This note was uploaded on 07/15/2011 for the course PHC 6000 taught by Professor Staff during the Summer '08 term at University of South Florida - Tampa.
- Summer '08