Class 14 Flashcards
(20 cards)
Cause
A precursor/event/condition/characteristic REQUIRED for the occurrence of the disease
–has to precede the disease
Associations
Relationships between an exposure and an outcome/disease
3 types of associations
1) Artifactual (aka False) associations
2) Non-causal associations
3) Causal associations
Artifactual associations
Can arise from bias and/or confounding
False associations
Non-causal associations
Can occur in 2 different ways:
A) The disease may CAUSE the exposure (rather than the exposure causing the disease)
-Ex: RA leading to physical inactivity
B) The disease and the exposure are both associated with a 3rd factor (confounding)
- Ex: The positive association shown between:
- Coffee drinking and CHD, or
- Down's syndrome & birth-order
Causal associations
Exposure——> Outcome
Types of causal relationships (associations)
1) Sufficient cause
2) Necessary cause
3) Component cause
1) Sufficient Cause
-A set of minimal conditions/events that inevitably produce disease
- 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” that collectively act to induce disease
**If it’s present, it’s sufficient enough by itself to cause disease
2) Necessary Cause
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– could be silent carrier or asymptomatic
**Necessary to be present if you’re going to get the disease. But isn’t sufficient on its own so if you have it, that doesn’t mean you are for sure going to get the disease
Component cause
AKA Risk Factor
Something that, if present/alive, 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
Some patients must be “primed” or “susceptible” to disease before Component Causes induce disease (multi-factorial)
**Majority of diseases fit in this category
Interactions in Causal Research: Synergism
Factors work together
-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- and environmental-factors acted together (in synergy), infants would only get the congenital disorder if exposed to BOTH factors
Both/all factors must be present all at the same time
Interactions in Causal Research: Parallelism
Factors work in parallel; “Either”
-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 gene- or environmental-factor but would not get the disorder if exposed to neither
Individually, they both cause disease. Without either, there is no disease.
Multiple causation
Multiple component-causes working in concert to collectively become sufficient causes
Example: CHD
Hill’s Criteria
Derived following U.S. Surgeon General’s 1964 report on smoking
“In what circumstances can we pass from this OBSERVED ASSOCIATION to a verdict of CAUSATION?”
Hill disagreed that “hard-and-fast” rules of evidence could be generated by which to judge likelihood of causation.
Hill’s Criteria (Guidelines) to know
Causal Inference Process– an interpretive, application process
1) Strength
2) Consistency
a) Specificity
3) Temporality
4) Biological Gradient
5) Plausibility
a) Coherence, Experiment, Analogy
*The higher number of criteria met, when evaluating an “association”, the more likely it MAY be causal
S–C–T–B–P
Some Cats Take Big Poops
Hill’s Criteria: Strength
Strength refers to the SIZE of the measure of association (RR/OR/HR)
The greater the association the more convincing it is that the association MIGHT be causal
Example: Smokers have up to a 20 times greater risk of developing lung cancer compared to non-smokers
**“A strong association is neither necessary nor sufficient for causality and weakness of an association is neither necessary nor sufficient for the absence of causality.”
Hill’s Criteria: Consistency
AKA Reproducibility
The repeated observations of an association in different populations under different circumstances in different studies (not just once!)
Example: Cause-effect relationship between cigarette smoking and CHD greatly strengthened by the fact that a large number of observational studies have consistently demonstrated an INCREASED risk
**Consistency may still obscure the truth
Hill’s Criteria: Temporality
The necessity that the cause PRECEDE the effect/outcome in time
Time-Order also describable:
- Proximate cause (short term interval)
- Distant cause (long term interval)
Example:
FACT: A number of studies have shown HIGHER lung cancer rates among former smokers during the first year after cessation than among those who continue to smoke
INFERENCE: Continuing to smoke must decrease the risk of lung cancer
ACTUALITY: A substantial portion of those who voluntarily stop smoking at any given time do so because of early symptoms and signs of the already-existing but as yet undiagnosed illness
Hill’s Criteria: Biologic Gradient
Present of a gradient of risk (dose-response) associated with the degree of exposure
Example: light smokers are 5 times more likely to develop lung cancer than non-smokers, whereas heavy smokers are 15times more likely than non-smokers.
Caution: some biologic factors demonstrate a threshold effect (i.e. No effect until a certain level of exposure is reached)
- lead exposure and mental retardation
- alcohol consumption and mortality
Hill’s Criteria: Plausibility
Presence of a biological feasibility to the association, which can be understood and explained (biologically/physiologically/medically)
Is the event/exposure biologically plausible, if really true?
At issue: Plausibility decision on criterion-based from existing/known beliefs, which may be flawed or incomplete
Example: Infection (H. Pylori) causes PUD
