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Exercise of causal inference

Liyan Guo

Department of Epidemiology

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Question1. In a large case-control study of pancreaticcancer,17% of the patients were found to be diabetic at thetime of diagnosis, compared to 4% of a well-matched

control group (matched by age, sex, ethnic group, andseveral other characteristics) that was examined fordiabetes at the same time as the cases were diagnosed. Itwas concluded that the diabetes played a causal role inthe pancreatic cancer. This conclusion:

A. Is correct B. May be incorrect because there is on control or

comparison group C. May be incorrect because of failure to establish the time

sequence between onset of the diabetes and diagnosis ofpancreatic cancer

D. May be incorrect because of less completeascertainment of diabetes in the pancreatic cancer cases

E. May be incorrect because of more completeascertainment of pancreatic cancer in non-diabeticpersons

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Question2. All of the following are important criteria whenmaking causal inferences except:

A. Consistency with existing knowledge

B. Dose-response relationship

C. Consistency of association in several studies

D. Strength of association

E. Predictive value

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Question3. Several studies have found that approximately85% of cases of lung cancer are due to cigarette smoking.

This measure is an example of:

A. An incidence rate

B. An attributable risk

C. A relative risk

D. A prevalence risk

E. A proportionate mortality ratio

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3,980204,000 Healthy

nonsmokers

1,935652,000 Healthysmokers

CHD Did Not

Developed

CHD

Developed

At Beginning of

Study

Outcome After 10 YRS

Questions 4 and 5 refer to the following

information:

The results of a 10-year cohort study ofsmoking and coronary heart disease (CHD)

are shown below:

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4. The incidence of CHD in smokers that

can be attributed to smoking is:

5. The proportion of the total incidence of

CHD in smokers that is attributable to

smoking is:

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The incidence of a disease that is attributable to the exposure

in the exposed group can be calculated as follows:

Incidence in exposed group - Incidence in nonexposed group(How much of the risk of the disease is due to exposure?)

The attributable risk as proportion of the total incidence in the

exposed group that is attributable to exposure can becalculated as follows:

(Incidence in exposed group - Incidence in nonexposed group)

Incidence in exposed group

(What proportion of the risk of the disease in exposed persons

is due to exposure?)

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Questions 6 and 7 refer to the followinginformation:

In a cohort study of smoking and lung cancer,the incidence of lung cancer among smokerswas found to be 9/1000 and the incidenceamong nonsmokers was 1/1000. From another

source we know that 45% of the total populationwere

smokers.

6. The incidence of lung cancer attributable to

smoking in the total population is: 7. The proportion of the risk in the total

population that is attributable to smoking is:

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Formula forAttributable Risk for the Total Population-Population Attributable Risk (PAR)

Incidence in total population-Incidence in nonexposed group

OR (Incidence in smokers) (%Smokers in population)

+(Incidence in nonsmokers) (%Nonsmokers in population)

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Formula for the attributable risk for the total

population

1)1(

)1(%

+

=

rp

rpPAR

Wherep is the proportion of the population with the

characteristic or exposure and r is the relative risk (or

odds ratio).

OR

(Incidence in totalpopulation- Incidence in nonexposed

group)

Incidence in total population