Case Control and OR

Question 1

Case-Control Study

Answer 1

Retrospective study where an investigator compares proportion of cases exposed to a risk factor with proportion of controls exposed to a risk factor.

Question 2

Case-Control Design

Answer 2

• Cases: subjects with the disease
• Controls: subjects without the disease
• Investigators look backward in time to identify the presence or absence of exposure/risk factors among cases and controls

Question 3

Case-Control Advantages

Answer 3

• Good for disease with long latency
• Cost and time efficient
• Good for rare diseases
• Allows evaluation of multiple risk factors

Question 4

Case-Control Disadvantage

Answer 4

BIAS

• Selection bias
• Information Bias: recall, misclassification, interviewer
• Do not allow calculation of incidence and RR

Question 5

Hospital-based Cases

Answer 5

• Subjects with the disease identified through a medical treatment facility
• Relatively easy, inexpensive

Question 6

Population-based Cases

Answer 6

• Select all persons with the disease or select a random sample from the general population
• Avoid biases arising from use of a particular medical facility
• More difficult logistically and more expensive

Question 7

Hospital Controls

Answer 7

Patients in same hospital being treated for diseases other than cases

Question 8

General Population Controls

Answer 8

Random-digit telephone dialing, voting lists, population registers

Question 9

Special Controls

Answer 9

Friend, neighbors, relatives of cases

Question 10

Hospital Control Advantages

Answer 10

• Easily identified and readily available
• Comparable to cases in their accuracy in reporting past exposure
• Subject to the same intangible selection factors that influence admission to a particular facility
• More willing to cooperate

Question 11

Hospital Controls Disadvantages

Answer 11

• Differ from healthy individuals

- Might not accurately represent the exposure distribution in the general population

Question 12

General Controls Advantages

Answer 12

-Assures the greatest level of comparability to cases

Question 13

General Controls Disadvantages

Answer 13

• Costly and time consuming
• Harder to contact
• Harder to recruit
• People who can be contacts during the day may be systemically different
• Potential recall bias

Question 14

Special Controls Advantages

Answer 14

• Healthy individuals
• More likely to cooperate
• Share similar risk factors as cases - control for confounding

Question 15

Special Controls Disadvantages

Answer 15

-Share similar risk factors: underestimation of association

Question 16

Measures of Association

Answer 16

• Can ONLY calculate odds ratio
• Odds ratio: ratio between the odds of exposure in the cases and the odds of exposure controls
• OR: odds of exposure among cases/odds of exposure among controls
• OR: AD/BC
• OR < 1 = protective effect
• OR = 1: no association
• OR > 1 = positive association

Question 17

Matching

Answer 17

• Process of selecting controls so that they are similar to the cases in certain characteristics
• Group matching: matching certain characteristics (age, gender, etc.)
• Individual: should a control for each case on a matching basis

Question 18

Matching Advantages

Answer 18

– Straightforward
– Effective
– Necessary for factors for which there would otherwise be insufficient overlap between the study groups in a random sample

Question 19

Matching Disadvantages

Answer 19

– Difficult, expensive, and time-consuming to find a comparison subject
– Limits sample size
– Require a special type of statistical analysis (McNemar’s, conditional logistic regression)
– Cannot evaluate the effect of the matched factors

Question 20

Evaluating Case-Control Studies

Answer 20

• Were the cases and control from a common population
• What standard was used to defined cases
• Are the controls truly free of the outcome
• How was exposure determined, was same applied for case and control
• Were potential confounders taken into consideration