Module 5: Case Control Flashcards
Attributable Proportion (AP)
- The fraction of exposed cases that would be
eliminated if the exposure were eliminated or
reduced. - A measure of the public health impact of an exposure.APe (exposure group) = [(OR - 1)/OR] x 100
APt (total pop.) = [(Pe)(OR - 1)]/[(Pe) (OR - 1) +1] x 1000
Disease Odds Ratio
The odds of being a case (a/b) among the exposed divided by the odds of being a case among the nonexposed (c/d):
(a/b)/(c/d)
Exposure Odds Ratio
The odds of being exposed among the cases (a/c) divided by the odds of being exposed among the controls (b/d):
(a/c)/(b/d)
Controls (In case-control studies)
- A sample of the population that gave rise to the cases
- Purpose is to provide information on the exposure
distribution in the source population
Case-Control Study
Type of observational study involving a group of individuals who have the outcome of interest (cases) and a group of individuals who are similar to the case individuals but do not have the outcome of interest (controls).
The researcher then looks at historical factors to identify if some exposure(s) is/are found more commonly in the cases than the controls.
Survivor Sampling
Controls sampled from survivors at the end of follow-up (noncases at the end of the observation period)
The predominant method of selecting controls in traditional case-control studies
Base Sampling
Controls sampled from population at-risk when follow-up begins (pop. at-risk at beginning of observation period)
Risk Set Sampling
Controls sampled from the population at risk as cases are diagnosed (pop. at risk when each case is diagnosed)
What relative measure of comparison is being estimated by the odds ratio when survivor sampling is used?
The odds ratio in the base population
What relative measure of comparison is being estimated by the odds ratio when case-based sampling is used?
The risk ratio of the base population
What relative measure of comparison is being estimated by the odds ratio when risk set sampling is used?
The rate ratio of the base population
Case-Crossover Study
- A new variant of the case-control study in which cases
serve as their own controls - Used in situations when brief exposure causes a
transient change in risk of a rare acute-onset disease - The exposure frequency during a hazard period is
compared with that during a control period
Hazard Period
In a case-crossover study, the period of increased risk following the exposure
Key Aspects of Case-Crossover Studies
- Purpose is to study the effect of transient exposures
on the risk of acute events - Cases serve as their own controls
- The brief period of increased risk following a
transient exposure is termed the hazard period - The exposure frequency during the hazard period is
compared to a control period
Important Considerations in Case Definition and Selection
- Criteria for case definition should lead to accurate
classification of diseased and nondiseased subjects - Efficient and accurate sources should be used to
identify cases - Incident cases are preferable to prevalent ones for
causal research - Partial case ascertainment is legitimate as long as
the source population can be defined
Advantages of case-crossover studies over traditional case-control studies
- Immune to control selection bias
- Cases and controls have many identical
characteristics (such as race and gender) - More efficient and require fewer subjects because
variability is reduced
Applications of Case-Control Studies
- evaluating the effectiveness of vaccines
- evaluating treatment and prevention programs
- investigating outbreaks of disease
Odds Ratio
- Odds of being a case among the exposed compared with the odds of being a case among the nonexposed,
or - Odds of being exposed among the cases compared with the odds of being exposed among the controls
OR = [a/b]/[c/d] = [ad]/[bc]
TROHOC
a disparaging term for a case-control study
The TROHOC Fallacy
means that it is incorrect to consider the logic of a case-control study backwards (from effect to cause) because the key comparison is identical to that of a cohort study: between exposed and unexposed groups
Risk
the chances of something happening/the chances of ALL things happening
Odds
the chances of something happening/the chances of it NOT happening
Strengths of Case-Control Studies
- Efficient for rare diseases and diseases with long
induction and latent period - Can evaluate many risk factors for the same disease
so good for diseases ab which little is known
Weaknesses of Case-Control Studies
- Inefficient for rare exposures
- Vulnerable to bias because of retrospective nature
of study - May have poor information on exposure
- difficult to infer temporal relationship between
exposure and disease
