Why are case control studies called "retrospective studies"
- study starts with end point / disease
- goes backward to identify risk factors
- no follow up of cases
Meaning of Odds Ratio
- approximation of the relative risk of disease in those exposed to risk factor compared to those who were not exposed to risk factor
- odds of exposed person having disease / odds of unexposed person having disease
Selection of Cases & Control
- controls à sourced from hospital catchment area / patients attending same hospital for treatment of other diseases
- match for age / sex
Data Collection
- pre-designed questionnaire à preferred as medical records are often incomplete
Differential Medical Surveillance
- biased because sub clinical cases would have gone undetected in community
Differential Classification
- questions about exposure from clinician depend on the disease being treated
Relative Risk and Odds Ratio
- RR – measures likelihood of developing disease in exposed group relative to those who are unexposed.
- OR – compares odds of exposure to risk factor among cases with that among controls.
Absolute & Relative Risk
- absolute risk – risk of having disease
- RR – risk of disease in exposed group relative to risk of disease in unexposed group.
o RR = 3.0 à risk of outcome in exposed group is 3 times that of unexposed group.
Incidence rate & Case Control Study
- case control study does not allow calculation of incidence
o subjects not carried fwd in time
o proportion of cases controlled by study design
Exposure Measurement
- breast feeding history obtained through interview
o recall bias
OR
- OR of 0.72 à women with a history of breastfeeding have a 28% lower risk of developing cancer than women without a history of breastfeeding.
- breast feeding has a protective effect
Case Control
- Before it is accepted that breast feeding provides protection …. results of study would need to be reproduced by other larger studies in a range of population groups
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