Case Control Studies

Question 1

What do case control studies involve?

Answer 1

• identify individuals with a disease (cases)
• identify similar individuals without the disease (controls)
• determine previous exposure
• relate information on exposure to disease

Question 2

How do you select cases for a case control study?

Answer 2

• representative of all people with the selected disease of interest
• could be incident cases from disease registry
• could be hospital based recruitment but may be biased

Question 3

How do you select controls?

Answer 3

• same population as cases
• if hospital based ensure reason for being in hospital not related to exposure
• usually more than 1 control per case

Question 4

Why and how should cases and controls be matched?

Answer 4

• know about potential confounders (age/gender)

- if using GP register can select by age and sex

Question 5

What is undermatching?

Answer 5

• cases and controls not similar enough

- different smoking ages

Question 6

What is overmatching?

Answer 6

• cases and control too similar

- sibling testing

Question 7

What are the types of bias in case control studies?

Answer 7

• recall bias: cases may remember more than controls
• reverse causality: disease caused changes in recent exposures
• selection of cases: representative of all people with disease
• selection of controls: representative of all people with disease

Question 8

What are nested case control studies?

Answer 8

prepared case control studies

• collected blood samples from larger population, frozen, then wait to see who gets disease
• cohort with disease split into exposed and unexposed
• cohort without split into exposed and unexposed

Question 9

What are the advantages of nested case control studies?

Answer 9

• cheap, quick, easy
• exposure before disease
• can retrieve stored samples to look at new biomarkers

Question 10

What are the disadvantages of nested case control studies?

Answer 10

• need cohort study with stored serum samples

Question 11

Why can you not use relative risks when analysing case control studies?

Answer 11

• do not know the risk of the disease as you start with cases with the disease

Question 12

What are odds ratios?

Answer 12

• used to analyse case control studies
• odds exposure in cases/odds exposure in control
• good estimate of relative risk if disease is rare
• if ratio is 2 = babies who died twice as likely to have been put on their sides rather than their backs compared to babies who did not die

Question 13

What is odds exposure in cases?

Answer 13

Number of exposed cases x number of unexposed controls

Question 14

What is odds exposure in controls?

Answer 14

number of exposed controls x number of unexposed cases

Question 15

What is absolute excess risk?

Answer 15

Risk in exposed - risk in unexposed

Question 16

What is attributable proportion?

Answer 16

incidence in population attributable to exposure/incidence in population

Question 17

What is a cross sectional study?

Answer 17

• measure existing disease and current exposure

- sample at 1 point in time without knowledge of disease or exposure

Question 18

What are the advantages of a cross sectional study?

Answer 18

• can look at exposures that won’t change (gender)

- gives measures of prevalence and exposure rates

Question 19

What are the disadvantages of cross sectional studies?

Answer 19

• no use for rare exposures or rare disease

- not useful for assessing casualty

Question 20

What is the risk of bias in different types of studies?

Answer 20

BIAS LIKLEY
- cross sectional
- case control
- cohort
- clinical trials
BIAS NOT LIKELY

Question 21

What is the strength of proof in different types of studies?

Answer 21

WEAK
cross sectional
case control
cohort
clinical trials
STRONG

Question 22

What is the Bradford Hill criteria for causation?

Answer 22

• strength of association
• dose response
• time sequence
• reversibility
• biological plausibility
• coherence of evidence in other study types
• consistency of findings in other populations

Question 23

Why may exposure and disease be associated?

Answer 23

• bias
• chance
• casual
• incorrect analysis
• confounding
• reverse causality