Association to Causation

Sequence of Studies

Clinical observations

Available data

Case-control studies

Cohort studies

Randomized trials

Types of Associations

• Real

• Spurious

DEFINITIONS

• Observational study

• Causation

• Association

• Necessary

• Sufficient

Koch and Causation

• Postulates

• Why study association

• Web of causation

• Do we need a better way

Twelve Criteria for Causation

• Cause distributed at same level

• Incidence much higher in exposed population

• Exposure more frequent

• Disease should follow exposure

• Dose dependent• Expected response

• Association should be the same

• Other cause-effect ruled out• Control results in decreased

disease • Modification of host results

in decrease• Human vol. always +• Findings should make

sense

Factors in Causation of Disease

• Predisposing

• Enabling

• Precipitating

• Reinforcing

Web of Causation

Are Associations Always Connected to the Disease?

NO, BUT …………………………..Cigarette smoking and lung cancerAge and prostate cancerCar accidents and alcoholTribal customs and kuruAgriculture and antibiotic resistance

Association

• Deals with ……………….

• Is concerned with ……….

• Is the degree of ………….

• Has to be scientifically proven …..

Your Assignment: fill in the blanks

Degrees of Association

• No assoc. Direct assoc.No possibility for associationPossibly associatedAssociatedDirect cause and effect

Guidelines for Judging Whether An Association Is Causal

• Temporal relationship• Strength of the association• Dose-response relationship• Replication of the findings• Biologic plausibility• Consideration of alternate explanations• Specificity of the association• Consistency with other knowledge

Deriving Causal Inferences

• Arriving at causation from association For example, showing that Helicobacter

pylori is directly linked to peptic ulcers

Perspectives

• Causation may be limited

• Subject to modification

• Perhaps more complex than realized

• Sometimes not measureable

• “Criteria” are really guidelinesmay be subjective