Lecture 20: Case control studies

Question 1

What are the steps for a case control study?

Answer 1

1. Identify Source Population
2. Identify people with outcome (cases)
3. Sample people without outcome from same population (controls)
4. Measure exposure prior to outcome in cases and controls
5. Compare odds of exposure to calculate measure of association (odds ratio)

Question 2

Why can’t you calculate incidence in a case control study?

Answer 2

Because you select the number of cases and controls and compare them (with/without outcome)

Question 3

How do we use odds for case control studies?

Answer 3

By measuring how many times as likely cases are to have the exposure compared to controls

This can help determine if the exposure is a risk factor

Question 4

How do we interpret Odds ratio?

Answer 4

People with ‘outcome’ are ‘x’ times as likely to have had the ‘exposure’ than people without the outcome

Question 5

What is important to remember in control selection for these case control studies?

Answer 5

Need to represent the odds of exposure in people without the outcome in the source population

Must be capable of becoming a case

Question 6

What is Cohort study vs Case control study

Answer 6

Cohort study = Ascertain exposure status, then find out outcome(s

Case control = Ascertain outcome status, then find out exposure(s)

Question 7

What are the strengths and limitations of case control studies?

Answer 7

Strengths:
- Can use Rare outcomes, transient exposures

• Can determine/use Multiple exposures
• Allows for Temporal sequencing
• Often comparatively quick and inexpensive

Limitations:
- Usually can only study one outcome

• Difficult to select appropriate control group
• Can be susceptible to selection and recall bias

Question 8

What is the purpose of a control group?

Answer 8

to estimate the prevalence of exposure in the population from which the cases came from