## Experimental & Observational, (Analytical, Descriptive)Study Designs

• COHORT STUDY
• compare the exposure (exposed vs unexposed)
• to find probability of getting disease (or disease incidence).
• Risk Ratio of ‘disease’ incidence.
• CASE-CONTROL STUDY
• compare the disease (diseased vs healthy)
• to find risk factor frequency.
• Odd Ratio of ‘exposure/risk factor‘.
• NESTED CASE CONTROL STUDY (NCC)
• A case-control study nested within a cohort study.
• exposure of interest is only measured among the cases and the selected controls.
• Because the number of cases is probably fairly small, can match multiple controls to a given case to increase the power.
• NCC design is often used when the exposure of interest is difficult or expensive to obtain and when the outcome is rare.
• NCC is more efficient than the full cohort design. Cases of a disease that occur in a defined cohort are identified and, for each, a specified number of matched controls is selected from among those in the cohort who have not developed the disease by the time of disease occurrence in the case.
##### Probability vs. Odds
• Probability is essentially the same as percentage.
• We tend to compare the number of occurrences of a certain outcome to the number of total events measured.
• Probability ranges between zero and one.
• Odds is a ratio of the likelihood of an event happening compared to the likelihood of an event not happening.
• Odds can be zero or any positive number (not just values between 0 and 1).
###### How big should the RR or OR ?
• In RCT we are satisfied with a small increase (or decrease) in risk.
• Cohort study: RR > 3 (for a minor adverse event)
• Case-control: OR > 4