Association and Causation

Question 1

If we find an association does that mean the exposure caused the outcome?

Answer 1

No

Question 2

Why is cause and effect not that simple?

Answer 2

• outcomes often have multiple causes
• sometimes certain things have to happen together and/or in a certain order for an outcome to occur

Exposure 1 + Exposure 2 + Exposure 3 = Outcome

Question 3

define a cause

Answer 3

an event, condition or characteristic [or a combination of these factors] that plays an essential role in producing an occurrence of the disease

Question 4

Describe the components of the causal pie model

Answer 4

This model is a concept that can help to explain te different types of causes, and helps with thinking about preventions
Whole ‘pie’ = sufficient cause
- Together, these exposures are sufficient to cause the outcome
- So we call the whole pie a sufficient cause of the outcome
- Each exposure is a component of the sufficient cause
- So we call each of the exposures a component cause

Shows the components of a sufficient cause for an outcome

Question 5

For many health conditions there are multiple sufficient causes, how do pie models help here?

Answer 5

Can look at the sufficient causes separately and see what makes them up.
- if there is the same component cause in multiple of the sufficient cause pie things then by eliminating that component cause you can get rid of a percentage of the cases of that disease
- the component cause that is in the most sufficient causes should be the one you eliminate first because it will get rid of more of the cases of disease
- the totals of the component causes together will make up more than 100% most of the time because they can be part of more than one sufficient cause

eg. Suf. cause 1 = 50% of the cases of disease
Suf. cause 2 = 30% of the cases of disease
Suf. cause 3 = 20% of the cases of disease
If the same component cause is in Suf cause 1 and 2 and you eliminate it, then you get rid of 80% of the cases of disease

Question 6

describe a necessary cause: the prevention ideal

Answer 6

• a component cause which is necessary for the disease to occur
• it must be part of every sufficient cause
• if you eliminate it, then you eliminate all cases of disease (because you get rid of all the sufficient causes)

Question 7

describe the complexity of causes

Answer 7

• reality is often complex
• different combination of exposures may lea to the same outcome
• the cause of a disease may not be the last thing that preceded it

Question 8

how do we determine causation?

Answer 8

• needs to be an association
• but that doesn’t necessarily mean it is casual, still need to consider is the association a valid association (or is it sure to chance, bias or confounding?)
• how do we determine if something is causal? judgement is required (using the guidelines - which are used to make a judgement based on the TOTALITY of evidence, they are not absolute requirements to be met)

Question 9

Describe guideline 1. Biological plausibility

Answer 9

Is there a plausible mechanism for the association (doesn’t necessarily mean on the cellular level)?
- (but epidemiological knowledge may precede knowledge on biological mechanisms)

Question 10

Describe guideline 2. experimental evidence

Answer 10

Is there evidence from human RCTs or animal experiments?
- however animal studies might not apply to humans

Question 11

Describe guideline 3. Specificity

Answer 11

Is the exposure specifically association with a particular outcome but not others>
- however, it is quite common for exposures to be related to many outcomes (and vice versa)

**don’t confuse this with specificity in screening

Question 12

Describe guideline 4. temporal sequencing

Answer 12

for something to be causal, the exposure must come BEFORE the outcome

Question 13

Describe guideline 5. consistency

Answer 13

are the findings consistent with findings from other studies?
- however, there can be a number of reasons why studies might have different findings (so up to your judgement but must explain)

Question 14

Describe guideline 6. dose-response relationship

Answer 14

Does the risk of the outcome change with increasing or decreasing amounts of the exposure?
- but not all relationships are linear (so sometimes can’t do this depending on what you are measuring)

Question 15

Describe guideline 7. strength of association

Answer 15

The stronger the association, the less likely it is to be due to confounding or bias
- however, this is not always the case eg. if confounding is very strong

According to textbook:
RR of 2 could be considered moderately strong
RR of 5 could be considered as strong
- but even a relationship that is not strong could be important

Question 16

what is the acronym for remembering the guidelines?

Answer 16

BEST CDS

Question 17

describe the use of guidelines

Answer 17

Consider them all and then make a judgement based on the totality of evidence
- is there any other way of explaining the set of facts before us, is there any other answer equally, or more likely than cause and effect?