Epidemiology Module 8

What is a case control study?

• A case-control study is a study in which a group of persons with a disease (cases) and a comparison group of persons without the disease (controls) are compared with respect to their history of past exposures to factors of interest

How are the groups chosen and compared in case control studies?

• Study population selected on the basis of the presence or absence of the disease or outcome in question

• Break participants into two groups:

  • Cases (with disease)

  • Controls (without disease)

• Groups are compared to determine the presence of specific exposures or risk factors

• Use interviews, records, surveys, or assay of biologic specimens to determine past exposures

Selection of Cases

• Establish a case definition

• Ensure homogeneity of cases

• Identify source population from various records

• Ensure cases are representative of the general population

Selection of Control

• Should stem from the same population as the cases

• Meet same inclusion criteria as the cases (minus the disease)

Matching

• Each case is paired with control that is similar in demographic and background variables

Selection Bias

• Mismatch of subjects

Measurement Bias

• Error that occurs due to poor measurement (classification) of study variables

  • Non-differential: misclassification between groups is approximately equal

  • Differential: misclassification differs between groups

Observer Bias

• Error that comes from the process of observing and recording

Recall Bias

• Error that occurs when participants have issues with remembering things correctly → self-reporting issues

Confounding Bias

• Distortion of the exposure-disease/outcome association

• Confusion or mixed effects that were not accounted for

• May compete with the exposure of interest in explaining the outcome

Confounder Properties

• A confounder must be associated with the exposure under study

• Confounder must also be a risk factor for the disease

• Confounder cannot be intermediate variable in the causal pathway between the exposure of interest and the disease

  • If the variable is caused by the exposure → intermediate variable

    • Ex: smoking → lung disease (intermediate) → lung cancer

  • If the variable causes both the exposure and the outcome and is not caused by the exposure → confounder

    • Ex: exposure is coffee drinking, outcome is lung cancer, confounder is smoking

Analysis of a Case Control Study

Odds Ratio

• Exposure odds ratio

  • The ratio of the odds of the exposure among cases divided by the odds of the exposure among the control

  • (a/c)/(b/d) → ad/bc

• Disease odds ratio

  • The ratio of the odds of disease among the exposed divided by the odds of disease among the non-exposed

Interpretation of Odds Ratio

• OR =1 → risk in exposed = risk in non-exposed (no association)

• OR > 1 → risk in exposed > risk in non-exposed (positive association)

  • Larger the OR: stronger the association

• OR < 1 → risk in exposed < risk in non-exposed (negative association)

  • Smaller the OR: larger the association

Advantages of Case Control Studies

• Quick and less expensive than prospective cohort studies

• If exposures of interest are relatively common, require a much smaller sample size than cohort studies

• Minimal ethical issues

• Useful for studying rare outcomes or those with long latency periods

• Multiple exposures or risk factors can be examined for a single outcome

Disadvantages of Case Control Studies

• Not as reliable

  • Susceptible to biases

• Cause and effect

• Less useful for rare risk factors

• Cannot confirm different levels or types of disease

• Control of extraneous variables is limited

• Finding comparison groups can be difficult

• Cannot determine the rate of disease