6.2 principles

1. Principles
• What do we do in a cohort study?
• How might we sample in a cohort study?
Szklo Figure 1-18
(modified)

2. Principles
• Study base principle
– Primary vs. secondary
• Primary study base
– Base can be defined as a specific population in space and
time
– Conceptually, the study base is identified first as the
population of interest for study
• Example: geographic area over particular time period
• Note: order of data collection not relevant to whether
study base is primary or secondary
– All cases of disease that arise from that population are the
cases
– Challenge is to completely identify all cases of disease

3. Principles
• Primary study base
– Example: population of Oakland, CA in 2010
– Example: members of Kaiser Permanente in the Bay
Area 1990-2000

4. Principles
• Secondary study base
– Base that can only be defined with respect to the
cases (cannot be clearly defined in space/time)
– Conceptually, the cases are identified first and they
define the study base
• Example: all patients treated for a gun shot wound at the
trauma center at Highland Hospital in Oakland, CA in 2010
– The study base is then all individuals who would have
been identified as cases if they had developed
disease during the study period

5. Principles
• Secondary study base
– Challenge here is to identify the study base
• Example: base is all individuals who would have been treated
for a gun shot wound at Highland Hospital in Oakland, CA in
2010 had they suffered one

6. Principles
• Deconfounding principle
– Important but not unique to case-control designs
– Confounding should not distort the measure of
association
– Measured confounders should be controlled

7. Principles
• Deconfounding principle
– Unmeasured confounders should vary as little as
possible – relates to matching (will cover this in
matching)
• Example – sibling controls will reduce genetic variability