Lecture 8: Causation in Epidemiology

Question 1

What are other ways to refer exposures by?

Answer 1

Causes, risk factors, independent variables

Question 2

What are other ways we think about outcomes?

Answer 2

effects, diseases, injuries, disabilities, deaths, dependent variables

Question 3

Statistical association versus biological causation

Answer 3

For causation, we need statistical association (how much is this affection the population) and the biological mechanisms. The presence of statistical association alone does not necessarily imply a causal relationship.

Question 4

Association (relationship)

Answer 4

Statistical dependence between two or more events, characteristics, or other variables.
Ex. as one increases/decreases, what happens to the other variable.

Question 5

Causality (causation / cause-effect relationship)

Answer 5

relating causes to the effects they produce.

Question 6

Cause

Answer 6

An event, condition, characteristic (or a combination of them) which plays an important role/predictable change in occurrence of the outcome.

Question 7

Deterministic Causality

Answer 7

cause closely related to effects, as in “necessary”/”sufficient’ causes

Question 8

Necessary cause

Answer 8

Has to be present in order to produce the outcome

Question 9

Sufficient cause

Answer 9

inevitably initiates or produces an effect. Sufficient, is when the factor can produce the outcome by itself. Includes “component causes.”

Question 10

Component Causes

Answer 10

Together they constitute a sufficient cause for the outcome in question

Question 10

Component Causes

Answer 10

Together they constitute a sufficient cause for the outcome in question

Question 11

Probabilistic Causality

Answer 11

Weak relationship, neither necessary nor sufficient.

Question 12

Effect Measures/Impact Fractions

Answer 12

• effect measures and impact fractions are closely related to the strength of association
• The higher the effect measures and population attributable risk, the more exposure is predictive of the outcome in question

Question 13

Necessary and Sufficient

Answer 13

• if the factor is present, the disease will always occur
• without the factor, the disease never develops
  only factor A -> Disease

Question 14

Necessary but Not Sufficient (Alone)

Answer 14

• each factor necessary but not in itself sufficient to cause the illness in itself, all are necessary to cause disease, but individually, none are sufficient to cause the disease.
• each risk factor alone cannot cause disease
• Thus multiple factors are required often in a specified temporal sequence

Question 15

Sufficient but not necessary

Answer 15

• that factor alone can produce the outcome but it is not the only factor that produces the outcome
• that outcome can still occur if that factor is not present

Question 16

Neither Sufficient Nor Necessary

Answer 16

• None of the risk factors are enough or alone to cause a disease
• There are multiude risk factors

Question 17

Henle-Koch’s Postulates

Answer 17

Four postulates should be met before a causal relationship between a disease agent and disease.
1. The agent must be present in every case
2. Agent must not be found in cases of other disease
3. Once isolated, the agent must be capable of reproducing the disease in experimental animals.
4. The agent must be recovered from the experimental disease produced.

Question 18

Bradford Hill’s Considerations for Causality

Answer 18

1. Strength of association
2. Consistency
3. Specificity
4. Temporality
5. Dose-response relationship
6. biological plausibility and coherence
7. Experiment-randomized controlled trials

Question 19

Strength of Association

Answer 19

• The larger an association between exposure and disease, more likely to be causal
• Percival Pott’s study: scrotal cancer is 200 times more likely to those exposed to chimney soots

Question 20

Consistency

Answer 20

• multiple epidemiologic studies using various locations, populations, and methods to show consistent association
• over 100s studies that show smoking and lung cancer are associated

Question 21

Specificity of Outcome

Answer 21

• more likely to be causal when they are specific
• the exposure causes only one disease
• asbestos causes lung cancer

Question 22

Temporarilty

Answer 22

• Exposure must precede the onset of disease
• is low serum cholesterol a cause of colon cancer or is the early phase of colon cancer cause low cholesterol

Question 23

Dose-response relationship

Answer 23

• increase the dose/exposure, increases the response
• those with alcohol addiction die faster than those who consume less or abstain

Question 24

Biological plausibility and coherence

Answer 24

• align with pre-existing theory, biology, statistical support
• agree with biological or clinical data?
• Hepatitis B associated with liver cancer -> viral copies identified in cancerous liver

Question 25

Experiment

Answer 25

• does the removal of the exposure affect the frequency of the outcome

Question 26

Screening in Epidemiology

Answer 26

Process of identifying an unrecognized disease or defect by the application of tests, examinations, or other procedures.
Classifies asymptomatic people as likely or unlikely to have a disease or defect.

Question 27

Purpose of screening in epidemiology

Answer 27

Delay onset of symptomatic or clinical disease. Improve survival.

Question 28

Screening Criteria

Answer 28

• suitable disease
• suitable test
• suitable screening program

Question 29

Suitable Disease

Answer 29

• has serious consequences
• is progressive
• disease treatment must be effective at an earlier stage
• prevalence of the detectable pre-clinical phase must be high
• ex. breast cancer, cervical cancer, hypertension

Question 30

Suitable Test

Answer 30

• inexpensive
• easy to administer
• has minimal discomfort
• high level of validity and reliability
  *validity is more important than reliability

Question 31

Valid Test

Answer 31

• does what it’s supposed to do
• classify people with pre-clinical disease as positive
• people without pre-clinical disease as negative

Question 32

Reliable Test

Answer 32

Gives you same results on repetition

Question 33

How to measure test validity

Answer 33

sensitivity and specificity
valid test has high sensitivity and specificity

Question 34

Sensitivity

Answer 34

enables you to pick up the cases of disease
a / a + c = those that test positive / all with disease

Question 35

Specificity

Answer 35

enables you to pick out the no diseased people
d / b + d = those that test negative / all without disease

Question 36

Suitable Screening Program

Answer 36

Application of a specific test in a specific population for a specific disease

Question 37

Evaluation of Screening Program

Answer 37

Predictive Value of a Positive Test (PV+)
Predictive Value of a Negative Test (PV-)

Question 38

Predictive Value of a Positive Test (PV+)

Answer 38

a / a + b = number who test positive with disease/ number with positive result

Question 39

Predictive Value of a Negative Test (PV-)

Answer 39

d / c + d = number who test negative without disease / number with negative result

Question 40

Bias with evaluating a screening program

Answer 40

volunteer bias
lead-time bias
length bias

Question 41

Volunteer bias

Answer 41

• people who choose to participate in the screening program may be healthier or at a higher risk of developing the disease than those that don’t participate
• those with higher health seeking behaviours
  motivated to maintain health and wellbeing
• access to resources, opportunities, and networks

Question 42

Lead-time bias

Answer 42

• the amount of time by which the diagnosis was advanced due to screening
• that survival may erroneously appear to be increased among screen-detected cases simply because the diagnosis was made earlier in the course of the disease

Question 43

Length Bias

Answer 43

• less aggressive forms of a disease are more likely to be picked up in a screening program because they have a longer detectable pre-clinical phase
• less aggressive forms of disease usually have better survival