Causation

1
Causation

• Learning Objectives
• To be able to discuss Different Theories of
  causation
• Causation in Infectious vs. Chronic Disease
• Modern view of causation

2

• Jay M. Fleisher MS, Ph.D.
• Associate Professor, Nova Southeastern University
• Studied at
• Columbia University School of Public Health MS
  Epidemiology
• NYU Ph.D. Environmental Epidemiology/Biostat
  istics

3
Causation

• Two types of medical research
• Bench work
• Epidemiology
• Bench work usually describes the underlying
  biology of disease
• Epidemiology either tests the results of bench
  work on human populations or provides input to
  the biomedical scientist on what we still do not
  know

4
What does the term Causal really mean?
5
Example 1 - HIV and AIDS

• Epidemiology identifies new disease caused by
  defect in immune system
• Bench science identifies the infectious agent
• Epidemiological studies confirm that agent causes
  disease in humans
• Causation is proven

6
Example 2 - What Causes an MI

• Epidemiological studies combined with laboratory
  study identify risk factors
• Cigarette smoking
• Cholesterol
• Elevated blood pressure
• Stress
• Family history
• Obesity
• Etc
• Which of the above contribute the most risk
• What are the relationships between risk factors

7
Therefore

• The issue of causation is not as simple as it
  first appears
• Thus, the need for a unifying concept of causation

8
A Unifying Model of Causal Relationships
9
The 2 Components

• Sufficient Cause
• precedes the disease
• if the cause is present, the disease always
  occurs
• Necessary Cause
• precedes the disease
• if the cause is absent, the disease cannot occur

10
The 4 Models of Causal Relationships
11
1. Necessary and Sufficient
Disease
Only Factor A
Sickle Cell Anemia
Genetic factors
RARELY OCCUR
12
2. Necessary but Not Sufficient
Factor A

Factor B
Disease

Factor C
13
2. Necessary but Not Sufficient - Example
Initiation

Latent Period
Cancer

Promoter
14
3. Sufficient but Not Necessary
Factor A
Factor B
Disease

Factor C
15
3. Sufficient but Not Necessary - Example
Ionizing Radiation
or
Benzene
Leukemia

or
Electromagnetic Fields?
16
4. Neither Sufficient Nor Necessary
Factor A

Factor B
and/or

Disease
Factor C
Factor D
and/or

Factor E
Factor F
17
4. Neither Sufficient Nor Necessary - Example
Smoking

Cholesterol
and/or

MI
HBP
Fam. History
and/or

Stress
Obesity
18
Therefore

• Concept of Necessary vs. Sufficient Causes
  provides a theoretical framework for causation of
  all disease
• How do we actually assess whether a Risk Factor
  is indeed Causal

19
Criteria for Assessing Causation

• Temporal relationship
• Exposure precedes the disease
• Strength of the Association
• Measured by the Relative Risk ( either the Rate
  Ratio or the Odds Ratio)
• Dose-response Relationship
• As the dose of exposure increases the risk of
  disease also increases
• Example Cigarette Smoking and Lung Ca
• Replication of the Findings
• Results replicated in other studies
• Biologic plausibility
• Does the association fit with what we know about
  the underlying biology
• Sometimes we know little or nothing about the
  undelieing biology ( Black Box epidemiology
• Consistency
• Alternative explanation eliminated

20
Criteria for Assessing Causation

• Biologic plausibility
• Does the association fit with what we know about
  the underlying biology
• Sometimes we know little or nothing about the
  underlying biology ( Black Box
  epidemiology)
• Example Asbestosis and Lung Ca.. Only have
  theory of mechanism
• Consideration of Alternate Explanations
• If knowledge exists, rule out or make sure
  studies took into account
• Cessation of Exposure
• If exposure is reduced or eliminated Risk will
  decline
• Example Ex-Smokers
• Specificity of the Association
• A specific agent is associated with only 1
  disease
• OK for infectious agents but falls apart with
  many Risk Factors for Chronic Illness
• Example Cigarette Smoking associated with
  several diseases

21
Relevant Web Sites

• http//www.defendingscience.org/sites/default/file
  s/upload/Rothman-Greenland.pdf
• http//www.facmed.unam.mx/deptos/salud/censenanza/
  spiii/spiii/rothman.pdf