Lecture 14 Flashcards
Association
Relationship between exposure and outcome/disease
3 types of association
- Artifactual (false) associations
- Non-casual associations
- Causal associations
Artifactual associations
Can arise from Bias and/or Confounding
Non-causal associations
Can occur in 2 different ways:
A. The Disease may cause the Exposure (rather than the Exposure causing the Disease)
Example: RA leading to physical inactivity
B. The Disease and the Exposure are both associated
with a third factor (Confounding)
Example: The positive association shown between:
• Coffee drinking & CHD, or
• Down Syndrome & Birth-order
Causal associations
Exposure causes Outcome
Types:
- Sufficient Cause
- Necessary Cause
- Component Cause
Sufficient cause
o A set of minimal conditions/events that inevitably
produce disease
o A Cause which precedes a disease, and if present, the
disease will always occur
- Quite rare; apart from genetic abnormalities
- Sufficient causes can still have multiple, required
‘components’ (termed Component Causes; a.k.a. Risk
Factors)) that collectively act to induce disease
Necessary cause
o A Cause which precedes a disease and has the
following relationship with it:
Cause must be present for the disease to occur, yet the Cause may also be present without the disease occurring
• Example: Mycobacterium tuberculosis; a necessary cause for TB to be diagnosed, yet can be present in individuals without clinical symptoms of the disease
Component cause
- Component Cause (a.k.a. Risk Factor)
o Something that, if present/active, increases the
probability (or likelihood) of a particular disease
Example: High LDL levels are RF for AMI
Example: Smoking is a RF for lung cancer
o Some patients must be primed or susceptible to
disease before Component Causes induce disease (multi-factorial)
Interactions in causal research
- Synergism
- Parallelism
Synergism
Synergism (factors work together; BOTH)
o The biological-interaction of 2 or more component
causes such that the combined measure of effect is
greater than the sum of the individual effects
Example: If gene- & environmental-factors acted together (in synergy), infants would only get the congenital disorder if exposed to BOTH factors
- Factors additive in increasing risk
Parallelism
Parallelism (factors work in parallel; EITHER)
o The biological-interaction of 2 or more component
causes such that the measure of effect is greater if
either is present
Example: Infants would only get the congenital disorder if exposed to either the gene- or environmental-factor but would not get the disorder if exposed to neither
- Factors not additive in increasing risk
Hill’s criteria (guidelines)
- Strength
- Consistency
a. Specificity - Temporality
- Biologic Gradient
- Plausibility
a. Coherence, Experiment, Analogy
The higher the number of criteria met, when evaluating
an association, the more likely it may be causal
Strength
Refers to the size of the measure of association (RR/OR/HR)
o The greater the association the more convincing it is
that the association might actually be causal
Example: Smokers have up to a 20 times greater risk of developing lung cancer compared to non-smokers
- Strong association is neither necessary nor sufficient
for causality and weakness of an association is neither
necessary nor sufficient for absence of causality
Consistency
Consistency (a.k.a., Reproducibility) ʹ
- the repeated observations of an association in different populations under different circumstances in
different studies (not just once!)
o Example: Cause-effect relationship between cigarette
smoking & CHD greatly strengthened by the fact that a
large number of observational studies have consistently demonstrated an increased risk
- Consistency may still obscure truth
Temporality
Temporality is the necessity that the cause precede the effect/outcome in time - Time-Order also describable: o Proximate cause (short term interval) o Distant cause (long term interval)
