Case-control studies

Question 1

What is a case control study?

Answer 1

Studies which compare exposure level in a group of individuals who have a specific disease (case) and in another group of individuals without the disease (controls)

• Observe to see what happened

Question 2

What is the definition of a case?

Answer 2

Some one with the disease or problem - a case

Question 3

What is the definition of a control?

Answer 3

Must be individuals who would have been cases if had developed outcome

Question 4

Where are sources that can be used as controls?

Answer 4

Community based controls (preferable) :
- Electoral register
- Random digit dialling
- Primary care controls
- Spouse or family controls

Other disease controls:
- Hospital controls
- Cancer registry controls

Question 5

How is exposure measured?

Answer 5

• Measurement is performed retrospectively
• Data reported by subject / from records

Question 6

What are strengths of case-control studies?

Answer 6

• Relatively quick ( no long periods of follow
  up)
• Efficient for rare outcomes
• Allows examination of >1 exposure for 1
  outcome
• No loss to follow up bias

Question 7

What are weaknesses of case control studies?

Answer 7

• Temporal sequence unknown –> reverse
  causality (cannot be sure exposure came
  before outcome)
• Inefficient for rare exposures
• Prone to bias (selection and measurement -
  recall and interviewer)
• Confounding - other factors may explain
  association
• Can not calculate incidence rate

Question 8

What is the definition of a confounder?

Answer 8

A third variable that provides alternative explanation for observed association between exposure and outcome

Question 9

What can a regression model be used for?

Answer 9

Can be used to assess the strength of association between an exposure and outcome.

Question 10

What are the main types of regression models?

Answer 10

Continuous outcome –> Linear regression model –> Regression coefficient summary statistic

Binary outcome –> Logistic regression model –> Odds ratio summary statistic

Question 11

What is the equation for odds ratio?

Answer 11

Odds Ratio (OR) = odds in exposed group /
odds in unexposed group

Question 12

What are outcome must an odds ratio have?

Answer 12

Outcome must be binary

Question 13

How can you calculate the confidence interval for odds ratio?

Answer 13

Can calculate sample odds ratio from the data - but its the population rather than the sample
that is of interest

• So can estimate population odds ratio using
  sample odds ratio and use Confidence
  Interval to show precision of the estimate of
  population odds ratio

Question 14

What do the P values mean for odds ratio?

Answer 14

Smaller p values = stronger evidence against the null hypothesis

P values should be interpreted in terms of strength of evidence, not statistical significance

Question 15

What should you use to draw a conclusion?

Answer 15

A combination of the p value and confidence interval should be used to draw conclusions