Lecture 20: Case Control Studies

Question 1

What do case control studies do?

Answer 1

Ascertain outcome status, then find the exposures

Question 2

What is the difference between a cohort study and a case control study?

Answer 2

Cohort studies ascertain exposure status and then find the outcome whereas case-control studies ascertain the outcome status and then find the exposures

Question 3

Case control studies are what types of studies?

Answer 3

observational and analytic

Question 4

Case control studies address the issues associated with what other type of study?

Answer 4

Cohort

Question 5

Case control studies address the issues associated with another type of study. What are the issues?

Answer 5

• cohort studies can be inefficient with rare/slow to develop outcomes
• cohort studies can be inefficient with transient/acute exposures

Question 6

What are the steps to carrying out a case control study?

Answer 6

1. identify source population
2. identify participants with a known outcome status (cases)
3. sample people without the outcome from the same population (controls)
4. measure exposure prior to the outcome in cases and controls
5. compare odds of exposure to calculate the measure of association

Question 7

What measure of association can be calculated from a case-control study?

Answer 7

odds ratio

Question 8

How do you calculate the odds ratio?

Answer 8

odds of exposure (cases) divided by the odds of exposure (controls)

Question 9

If the odds of exposure (cases) is greater than the odds of exposure (controls) then the outcome is more likely to occur in the _____ and therefore the exposure is a potential _______ factor

Answer 9

cases

risk

Question 10

If the odds of exposure (controls) is greater than the odds of exposure (cases) then the outcome is more likely to occur in the _____ and therefore the exposure is a potential _______ factor

Answer 10

controls

protective

Question 11

What can not be calculated from a case control study?

Answer 11

incidence
relative risk
risk difference

Question 12

Why can we not calculate prevalence in a case control study?

Answer 12

because we have selected the number of people in the study with and without the outcome

Question 13

Why can we not calculate incidence in a case-control study?

Answer 13

Because the incidence in the controls would be zero and because we have selected the number of people in the study with and without the outcome

Question 14

In the box, what letter are used to calculate the odds of exposure in cases?

Answer 14

a/c

Question 15

In the box, what letter are used to calculate the odds of exposure in controls?

Answer 15

b/d

Question 16

What is the null value for odds ratio?

Answer 16

1

Question 17

What is the template for the interpretation of odds ratio?

Answer 17

we can just use the RR interpretation:

[exposed group] were [value] times as likely to [outcome] compared to [comparison group]

Question 18

Why can we use the relative risk interpretation for the odds ratio interpretation?

Answer 18

because when disease is are, OR approximates the RR

Question 19

If controls don’t have the outcome, how can we carry out step 4: measure exposure prior to the outcome in cases and controls?

Answer 19

by using index dates

Question 20

What are index dates?

Answer 20

We pretend that the control had the event on the same date as the case (or close to it)

Question 21

Case-control studies can only look at one _______

Answer 21

outcome

Question 22

When selecting the cases, do we look at prevalent cases or incident cases and why?

Answer 22

we look at incident cases because we only want to consider the development of an exposure and not the duration

Question 23

What two things must the control group be/do in a case-control study?

Answer 23

• they must be capable of becoming a case

- they need to represent the exposure distribution in a source population

Question 24

What is the purpose of the control group?

Answer 24

to estimate the prevalence of exposure in a population from which the cases come from

Question 25

What are the strengths of a case-control study?

Answer 25

• Good for rare outcomes and transient exposures
• Assess multiple exposures
• Temporal sequencing
• Quick and inexpensive

Question 26

What are the limitations of a case-control study?

Answer 26

• Only studies one outcome
• Hard to get an appropriate control group
• Prone to information bias