Causation

Question 1

Learning outcomes

Answer 1

1. To understand the difference between necessary and sufficient causes]
2. To be able to describe and apply Bradford Hill’s guiding criteria for causal assessment, and understand some of their shortcomings
3. To be aware of the distinction between causes at an individual level and those at a population level (and the significance of the distinction) .

Question 2

What is a causal factor?

Answer 2

Causal Factor•‘…event, act, condition or state of nature that “initiates or permits, alone or in conjunction with other causes, a sequence of events, resulting in an effect”’
•Counterfactual thinking
A factor is a cause if the outcome would not have occurred in the absence of that factor, holding all other things constant, including space and time.

Question 3

What is Rothman’s ‘pie’ model of causation?

Answer 3

See lecture on causation for diagrams
Rothman’s ‘pie’ model of causation
•A ‘sufficient’ cause is defined as a constellation of causes that together inevitably produces disease

•If a factor is a component cause in every sufficient cause, it is deemed a ‘necessary’ cause (i.e. factor A)

Question 4

What is the marbles analogy?

Answer 4

Collecting marbles across the life course: Collecting throughout life:

• Tobacco smoke in utero
• Chronic poverty, chaotic home environment in childhood
• Cigarette smoking starts in adolescence
• Poor nutrition in adulthood

Marbles” may not be independent
•Shared across individuals
•One person’s marble collection may influence another person’s marble collection
•Example, person-to-person infectious disease transmission

Examples of “shared marbles”
•Unhealthy food environment •Community violence
•Social norms around substance use and cigarette smoking; e.g., adolescents are more likely to begin smoking if an influential peer begins smoking •Policies and laws managing access to quality health care

Question 5

What are the public health implications of the marble example?

Answer 5

•Marble example, individual
oEach individual’sset of marbles that caused disease may be unique, with or without overlap across individuals
o Each marble was necessaryfor that person to develop the disease when and how he or she did
o Preventing any one of the marbles can prevent disease in that individual
•Marble example, population
o Epidemiologists look for the ‘marbles’ that are most common across individuals with disease compared to those without disease
o Preventing any one of the marbles can prevent disease in that individual
o “Preventing” the common marbles can prevent more disease in more individuals

Question 6

Bradford-hill criteria of causation

Answer 6

Bradford–Hill Criteria of Causation •Strength of Association •Dose-response relationship•Consistency •Temporal relationship•Intervention •Biological Plausibility •Specificity•Coherence •Analogy

See lecture for detailed explanations of each criteria