Case Control Studies

Question 1

What are the fundamentals of case control studies? [4]

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

Where do you source cases [3] and controls [3]

Answer 2

Cases:
* Want representative of all people with the disease of interest
* Could be incident cases from disease registry
* Could be hospital based recruitment but this may give a biased sample

Controls:
* same population as cases
* If hospital based ensure the reason for being in hospital not also related to exposure of interest
* Usually more than one control per case

Question 3

What is undermatching? [1]
Give an example [1]

What is overmatching? [1]
Give an example [1]

Answer 3

Undermatching: Cases and controls aren’t similar enough
* don’t match on age cases may be older and therefore more likely to smoke than controls.

Overmatching: Cases and controls may be too similar
* Eg if choose siblings may not differ in exposure of interest, for example parental smoking

Question 4

Name 4 causes of bias in case control studies

Answer 4

Recall bias
* Cases may remember more than controls

Reverse causality
* Has disease caused changes in recent exposures

Selection of cases
* Are they representative of all people with the disease

Selection of controls
* Are they representative of all people without the disease
* Are they similar to the cases

Question 5

Describe structure of a nested case control study [2]

Answer 5

“Prepared” the case control studies: collected e.g. blood samples from a large population.

These are frozen and then waited to see who gets the disease

.

Question 6

Nested case-control studies are good for assessing what? [1]

Answer 6

Useful for biomarkers studies where biomarker is expensive to measure

Question 7

I want to investigate a risk factor for a rare disease. Which should you use?
Why?

Cohort
Case control

Answer 7

Case control

If a disease is extremely rare a cohort study may have to be impractically large to get enough people with the disease

Question 8

I want to investigate if a rare exposure is associated with a disease ? Which should I use and why?

Cohort
Case control

Answer 8

Cohort

If an exposure is rare there may be too few people exposed in a case-control study to be able to draw conclusions

Question 9

Why cant you calculate relative risk of of a case control study?

Answer 9

Do not know risk of the disease (as you have started with cases with the disease)

Question 10

Whata can you assess instead of relative risk in a case control study?

Answer 10

Use Odds Ratios = odds exposure in case / odds exposure in controls

Question 11

State 4 causes of bias in case control studies [4]

Answer 11

1. Recall bias: cases may remember more than controls. If you have a disease and are being asked about previous exposures, you will have more thought into this than the controls will.
2. Reverse causality: has disease caused changes in recent exposures?
3. Selection of cases: are they representative of all people with the disease?
4. Selection of controls: are they representative of all people without the disease and are they similar to the cases?

Question 12

How effective is odds ratio for an estimate of relative risk if a disease is rare? [1]

Answer 12

If the disease is RARE the odds ratio is a good estimate of the relative risk

Question 13

Interpret this odds ratio [1]

Answer 13

Babies who died were twice as likely to have been put on their sides rather than their backs compared to babies who did not die

Question 14

How can you assess for confounding variables in controsl that weren’t matched for? [1]

Answer 14

logistic regression

Question 15

How can you assess for confounding variables in controsl that weren’t matched for? [1]

Answer 15

logistic regression

Question 16

absoulte risk in case control

Question 17

How do you calculate attributable proportion in a case control study?

Answer 17

Question 18

Calculate the attributable proportion for this data?

Answer 18

Question 19

Does exposure to blue asbestos cause lung cancer - What study should you do?

Case-control
Cohort
Clinical trial
Cross-sectional survey

Answer 19

Does exposure to blue asbestos cause lung cancer - What study should you do?

Case-control
Cohort: Rare exposure
Clinical trial
Cross-sectional survey

Question 20

Is prevalence of back pain more common in men or women ?

Case-control
Cohort
Clinical trial
Cross-sectional survey

Answer 20

Is prevalence of back pain more common in men or women ?

Case-control
Cohort
Clinical trial
Cross-sectional survey

Question 21

Place the following in order of most to least likely to have bias

Case-control
Cohort
Clinical trial
Cross-sectional survey

Answer 21

Question 22

Place the following in order of most to least likely to have a risk of confounding factors

Case-control
Cohort
Clinical trial
Cross-sectional survey

Answer 22

Question 23

Place the following in order the strength of proof in causility in the following?

Case-control
Cohort
Clinical trial
Cross-sectional survey

Answer 23

Question 24

Name 5 reasons, apart from causility that may have associations between an exposure and a disease [5]

Answer 24

Bias
Reverse causality
Confounding
Incorrect analysis
Chance

Question 25

What criteria needs to be met to assess if an association is causal? [1]

Answer 25

Bradford Hill Criteria for Causation

Question 26

Name and explain the 9 criteria of Bradford Hill Criteria of Causility [9]

Answer 26

**Temporal relationship:** Exposure always precedes the outcome **Strengh:** size of the the association as mesured by appropriate stastistical tests **Dose-response Relationship:** an increasing amount of exposure increases the risk **Consistency:** The association is consistent when results are replicated in studies in different settings using different methods **Plausibility**" the association agrees with currently accepted understanding of pathological processes **Consideration of alternate explanations** **Experiment**: the condition can be altered by an appropriate experimental regimen **Specificity:** this is established when a single putative cause produces a speficic effect **Coherence**: this association should be compatible with existing theory and knowledge

Question 27

How do you calculate odds exposure in cases and odds exposure in controls? [2]

Answer 27

**Odds exposure cases**: number of exposed cases x number of unexposed controls **Odds exposure controls**: number of exposed controls x number of unexposed cases

Question 28

What are relative and odds ratios like in case control studies when the disease is rare [1] and when disease is common? [1]

Answer 28

Relative risk and odds ratio are similar when disease is rare, but different when disease is common