Module 5: Case Control

Question 1

Attributable Proportion (AP)

Answer 1

• The fraction of exposed cases that would be
  eliminated if the exposure were eliminated or
  reduced.
• A measure of the public health impact of an exposure.APe (exposure group) = [(OR - 1)/OR] x 100
  APt (total pop.) = [(Pe)(OR - 1)]/[(Pe) (OR - 1) +1] x 1000

Question 2

Disease Odds Ratio

Answer 2

The odds of being a case (a/b) among the exposed divided by the odds of being a case among the nonexposed (c/d):
(a/b)/(c/d)

Question 3

Exposure Odds Ratio

Answer 3

The odds of being exposed among the cases (a/c) divided by the odds of being exposed among the controls (b/d):
(a/c)/(b/d)

Question 4

Controls (In case-control studies)

Answer 4

• A sample of the population that gave rise to the cases
• Purpose is to provide information on the exposure
  distribution in the source population

Question 5

Case-Control Study

Answer 5

Type of observational study involving a group of individuals who have the outcome of interest (cases) and a group of individuals who are similar to the case individuals but do not have the outcome of interest (controls).

The researcher then looks at historical factors to identify if some exposure(s) is/are found more commonly in the cases than the controls.

Question 6

Survivor Sampling

Answer 6

Controls sampled from survivors at the end of follow-up (noncases at the end of the observation period)

The predominant method of selecting controls in traditional case-control studies

Question 7

Base Sampling

Answer 7

Controls sampled from population at-risk when follow-up begins (pop. at-risk at beginning of observation period)

Question 8

Risk Set Sampling

Answer 8

Controls sampled from the population at risk as cases are diagnosed (pop. at risk when each case is diagnosed)

Question 9

What relative measure of comparison is being estimated by the odds ratio when survivor sampling is used?

Answer 9

The odds ratio in the base population

Question 10

What relative measure of comparison is being estimated by the odds ratio when case-based sampling is used?

Answer 10

The risk ratio of the base population

Question 11

What relative measure of comparison is being estimated by the odds ratio when risk set sampling is used?

Answer 11

The rate ratio of the base population

Question 12

Case-Crossover Study

Answer 12

• A new variant of the case-control study in which cases
  serve as their own controls
• Used in situations when brief exposure causes a
  transient change in risk of a rare acute-onset disease
• The exposure frequency during a hazard period is
  compared with that during a control period

Question 13

Hazard Period

Answer 13

In a case-crossover study, the period of increased risk following the exposure

Question 14

Key Aspects of Case-Crossover Studies

Answer 14

1. Purpose is to study the effect of transient exposures
   on the risk of acute events
2. Cases serve as their own controls
3. The brief period of increased risk following a
   transient exposure is termed the hazard period
4. The exposure frequency during the hazard period is
   compared to a control period

Question 15

Important Considerations in Case Definition and Selection

Answer 15

1. Criteria for case definition should lead to accurate
   classification of diseased and nondiseased subjects
2. Efficient and accurate sources should be used to
   identify cases
3. Incident cases are preferable to prevalent ones for
   causal research
4. Partial case ascertainment is legitimate as long as
   the source population can be defined

Question 16

Advantages of case-crossover studies over traditional case-control studies

Answer 16

1. Immune to control selection bias
2. Cases and controls have many identical
   characteristics (such as race and gender)
3. More efficient and require fewer subjects because
   variability is reduced

Question 17

Applications of Case-Control Studies

Answer 17

• evaluating the effectiveness of vaccines
• evaluating treatment and prevention programs
• investigating outbreaks of disease

Question 18

Odds Ratio

Answer 18

1. Odds of being a case among the exposed compared with the odds of being a case among the nonexposed,
   or
2. Odds of being exposed among the cases compared with the odds of being exposed among the controls

                 OR = [a/b]/[c/d] = [ad]/[bc]

Question 19

TROHOC

Answer 19

a disparaging term for a case-control study

Question 20

The TROHOC Fallacy

Answer 20

means that it is incorrect to consider the logic of a case-control study backwards (from effect to cause) because the key comparison is identical to that of a cohort study: between exposed and unexposed groups

Question 21

Risk

Answer 21

the chances of something happening/the chances of ALL things happening

Question 22

Odds

Answer 22

the chances of something happening/the chances of it NOT happening

Question 23

Strengths of Case-Control Studies

Answer 23

1. Efficient for rare diseases and diseases with long
   induction and latent period
2. Can evaluate many risk factors for the same disease
   so good for diseases ab which little is known

Question 24

Weaknesses of Case-Control Studies

Answer 24

1. Inefficient for rare exposures
2. Vulnerable to bias because of retrospective nature
   of study
3. May have poor information on exposure
4. difficult to infer temporal relationship between
   exposure and disease

Question 25

The measure of association between exposure and occurrence of disease in case-control studies

Answer 25

The odds ratio (OR)