1.6 Causation

Question 1

What is a cause?

Answer 1

An event, condition or characteristic that plays an essential role in producing an occurrence of the disease

Question 2

Can we prove causality?

Answer 2

Ultimately medical experiments alone can’t prove causality but rather suggest causality

Question 3

What is Causal Inference?

Answer 3

Judgements linking postulated causes and their outcomes.

Question 4

What are the three models for observing causal inference?

Answer 4

Epidemiological triads
Causal pies
Causal webs
These don’t allow you to test causality but rather allow you to model possible causal pathways and then go on to test them

Question 5

What are the four elements of the epidemiological triad?

Answer 5

Agent (carcinogen in smoke of tobacco)
Host (risk factors that influence susceptibility, exposure or response to agent)
Environment (extrinsic factors that affect the agent and the opportunity for exposure)
Vector (transmits agent to host)

Question 6

What are the 3 classifications of causal inference for exposure?

Answer 6

Sufficient Cause
Component Cause
Necessary Cause

Question 7

Define: Sufficient Cause

Answer 7

Factor (or often a combination) that will eventually produce a disease

Question 8

Define: Component Cause

Answer 8

Factor that contributes towards causation but not sufficient to cause disease on its own

Question 9

Define: Necessary Cause

Answer 9

Any agent that is required to develop a disease. Always precedes disease.

Question 10

What are the three levels of disease prevention?

Answer 10

Primary, Secondary and Tertiary

Question 11

Describe Primary Prevention

Answer 11

Prevent disease well before it develops. Reduce the risk factors in the whole population, selected groups and even healthy individuals. Primary care advice as past of routine consultation such as prevention programs.

Question 12

Describe Secondary Prevention

Answer 12

Early detection of disease (e.g. screening and intervention for pre-diabetes). Selected individuals at high risk (pre-symptomatic). Primary care risk factor reduction programs such as treating asymptomatic HIV patients with retrovirals.

Question 13

Describe Tertiary prevention

Answer 13

Treat established disease to prevent deterioration and limit disease progression. e.g. Exercise advice for obese patients.

Question 14

What are the Bradford-Hill criteria for determining causality?

Answer 14

Temporality
Biological Plausibility
Consistency
Strength of Association
Biological Gradient / Dose Response
Specificity
Coherence
Experimental
Analogy
Reversibility (not technically Bradford-Hill but often used on modern epi)

Question 15

Describe ‘Temporality’ as it relates to determining causality

Answer 15

Does the cause precede the effect or disease (HPV infection precedes cervical cancer)

Question 16

Describe ‘Biological Plausibility’ as it relates to determining causality

Answer 16

Is the association explained by a credible biologic mechanism (e.g Studies have shown biological plausibility through in-vitro & animal experiments)

Question 17

Describe ‘Consistency’ as it relates to determining causality

Answer 17

Can the results be replicated in different populations under different conditions (numerous studies have shown the same association between HPV and Cervical Cancer)

Question 18

Describe ‘Strength of Association’ as it relates to determining causality

Answer 18

Is the magnitude of the association large enough to rule out error due to chance. (RR=17.47. Infection with HPV increases your risk of cervical cancer 17fold.)

Question 19

Describe ‘Biological Gradient’ as it relates to determining causality

Answer 19

Does the frequency of disease increase with the dose of exposure? (Risk of cervical cancer may increase with viral load)

Question 20

Describe ‘Specificity’ as it relates to determining causality

Answer 20

Exposure associated specifically with single effect - not multiple effects. Unlikely to get one outcome –> one disease. More specific association –> higher probability of causal relationship. HPV types 16 & 18 specific to cervical cancer. HPV present only in tumour cells and not stromal cells.

Question 21

Describe ‘Coherence’ as it relates to determining causality

Answer 21

Results of study are not conflicting with current understanding and natural history of disease. Further studies are needed on either side of the debate to prove causality. Coherent with previous knowledge of natural history of HPV infection to cervical cancer described

Question 22

Describe ‘Experimental’ as it relates to determining causality

Answer 22

Is there positive experimental evidence? Often not available in human data. Animal models studies show evidence of specific papillomaviruses inducing papillomas and cancers in susceptible hosts

Question 23

Describe ‘Analogy’ as it relates to determining causality

Answer 23

Do other similar exposures cause similar outcomes?

HPV and cervical cancer model is similar to other cancers caused by viruses

Question 24

Describe ‘Reversibility’ as it relates to determining causality

Answer 24

Does the removal of the possible cause reduce the disease risk? E.g. cessation of cigarette smoking is associated with reduction in lung cancer risk. Furthermore, not smoking is associated with a decreased risk of lung cancer.

Question 25

Mnemonic For Remembering The Bradford Hill Criteria

Answer 25

The
Brave
Careful
Scientist
Became
So 
Concerned
Especially (over)
Associations &
Risks