Association & Causation

Question 1

Association

Answer 1

statistical dependence between 2 variables
Degree to which the rate of disease in persons with a specific exposure is either higher or lower than the rate of disease without that exposure.

Question 2

What to consider when evaluating statistical association

Answer 2

Chance
Bias
Confounding
Cause

Question 3

Why assess chance?

Answer 3

Inference may be made from samples rather than whole populations
Use stat. tests, find p values and confidence intervals

Question 4

What is bias?

Answer 4

A systemic error leading to an incorrect estimate of effect of an exposure on development of a disease or outcome of interest

Question 5

Eliminating bias?

Answer 5

bias = consequence of defects in design or execution of a study.
Can’t be controlled in analysis of study.
Can’t be eliminated by increasing sample size.

Question 6

What are the 2 broad types of bias?

Answer 6

Selection bias

Measurement bias

Question 7

What is selection bias?

Answer 7

A systematic difference between characteristics of people selected & those who weren’t.

Question 8

What is measurement bias?

Answer 8

When measurements/ classifications of disease/ exposure are inaccurate

Question 9

Give an example of selection bias

Answer 9

Healthy entrant effect

Question 10

Give an example of measurement bias

Answer 10

Recall bias

Question 11

Confounding

Answer 11

any factor believed to have a real effect on risk of disease under investigation & is also related to the risk factor under investigation

Question 12

Confounding includes factors

Answer 12

that have a direct causal link with the disease (e.g. smoking & lung cancer)
that are good proxy measures of more direct unknown causes (e.g. age & social class).

Question 13

Common confounders

Answer 13

Age
Sex
Socio-economic class
Geography

Question 14

Why is socio-economic class a confounder?

Answer 14

–Poorer people have higher rate of almost all disease.

–Higher risk of early death in poor people.

Question 15

Cause?

Answer 15

Judgement of a cause-effect relationship
based on 2 main areas:
1. Observed association between an exposure & a disease is valid
2. Totality of evidence taken from a number of sources supports a judgement of causality

Question 16

Bradford Hill Criteria for establishing causation

Answer 16

Temporal relationship
Plausibility
Consistency with other investigations
Strength of association
Dose-response relationship
Specificity
Experimental evidence
Coherence
Analogy

Question 17

What should you also consider when establishing causation

Answer 17

Reversibility

Question 18

Strength in BHC

Answer 18

Strong association more likely causal than weak association

Question 19

Consistency in BHC

Answer 19

Similar results found in different studies

Unlikely they were all subject to the same type of errors.

Question 20

Specificity in BHC

Answer 20

A particular exposure increasing the risk of a certain disease but not the risk of other diseases

Question 21

Temporal relationship in BHC

Answer 21

For a putative risk factor to be the cause of a disease it has to precede the disease.

Question 22

Which criterion of BHC is essential

Answer 22

Temporal relationship

Question 23

Dose-response relationship in BHC

Answer 23

Further evidence of a causal relationship if increased levels of exposure lead to increased risk of disease.
Some causal associations show a single jump (threshold) rather than a monotonic trend.

Question 24

Plausibility in BHC

Answer 24

Association is more likely to be causal if consistent with other knowledge e.g. animal experiments

Question 25

Experimental evidence

Answer 25

Experimental evidence on humans (rare) or animals.

Question 26

Coherence in BHC

Answer 26

Implies a cause & effect interpretation that doesn’t conflict with what is known of the natural history

Question 27

Analogy in BHC

Answer 27

provides a source of more elaborate hypotheses about association in question