1. Designs
• Risk-set sampling
What if a
selected
control
becomes a
case at a later
time?
Szklo Figure 1-20
JC: not every available control is used; a control can be used again later as a control or case
2. Designs
• Density sampled design
– Sampling approach to estimate the incidence density ratio
– Incident cases of disease are identified (numerator)
– Controls (non-diseased) are a sample from the study base that gave
rise to the cases, with sampling probabilities based on the person-time
of observation (denominator)
– This type of sampling of controls is called density sampling because OR
estimates IDR (we will go through that in a bit)
– Data are analyzed as matched on time (usually with conditional logistic
regression)
– Design often called a nested case-control study if it is carried out within
an actual cohort study
If you had a cohort study, why would you nest a case-control study
inside it?
• Even if not in an actual cohort study a density sampled case-control design
can be thought of as nested in an enumerated source population
3. Designs
• With our case-control designs we capture:
• With a cohort design we captured:
4. Designs
exposed
PTe d
b
• Risk-set sampling
c
a
PTu
If b/d = PTe/Ptu then the exposure
distribution in the controls represents
the exposure distribution in the total
cohort person-time
5. Designs
• Pseudo-rates
• Controls selected so that exposure distribution
among the controls is the same as it is in the
person-time in the study base for the cases
• This is met if:
– b/d = PTe/PTu
– b/PTe and d/PTu are control sampling fractions (r)
• Actual rates vs. pseudo-rates
– IDe = α/PTe Pseudo-ratee = a/b
– IDu = γ/PTu Pseudo-rateu = c/d
– Note: typically α=a and γ=c
6. Designs
• Compare the rates to the pseudo-rates
Multiply numerator by (PTe / PTe =1) and denominator by (PTu/PTu=1)
• This algebra “works” if “r” (the sampling fraction) is the same for the
exposed and unexposed
• In other words: This only “works” if cases and controls are selected so
that the exposure distribution among each group is the same as it is in the
person-time in the study base for each group...this is accomplished in the
design phase (and also the requires the proper analysis method--
conditional logistic regression)
