Q1) Submit Assignment: Case Study The following case study involves case-control and cohort studies involving smoking and lung cancer. Through this application, you will be able to explore the...

Centers for Disease Control and Prevention

Epidemiology Program Office
Case Studies in Applied Epidemiology
No. 731-703





Cigarette Smoking and Lung Cancer


Student's Guide

Learning Objectives
After completing this case study, the participant should be able to:

G Discuss the elements of study design, and the advantages and disadvantages of
case-control versus prospective cohort studies;

G Discuss some of the biases that might have affected these studies;

G Calculate a rate ratio, rate difference, odds ratio, and attributable risk percent;

G Interpret each measure and describe each measure's main use; and

G Review the criteria for causation.







This case study is based on the classic studies by Doll and Hill that demonstrated a relationship
between smoking and lung cancer. Two case studies were developed by Clark Heath, Godfrey
Oakley, David Erickson, and Howard Ory in 1973. The two case studies were combined into one and
substantially revised and updated by Nancy Binkin and Richard Dicker in 1990. Current version
updated by Richard Dicker with input from Julie Magri and the 2003 EIS Summer Course instructors.










U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service
CDC / EIS Summer Course 2003: Smoking and Lung Ca - Student's Guide Page 2


A causal relationship between cigarette smoking
and lung cancer was first suspected in the
1920s on the basis of clinical observations. To
test this apparent association, numerous
epidemiologic studies were undertaken between
1930 and 1960. Two studies were conducted by
Richard Doll and Austin Bradford Hill in Great
Britain. The first was a case-control study begun
in 1947 comparing the smoking habits of lung
cancer patients with the smoking habits of other
patients. The second was a cohort study begun
in 1951 recording causes of death among British
physicians in relation to smoking habits. This
case study deals first with the case-control
study, then with the cohort study.

Data for the case-control study were obtained
from hospitalized patients in London and vicinity


over a 4-year period (April 1948 - February 1952).
Initially, 20 hospitals, and later more, were asked
to notify the investigators of all patients admitted
with a new diagnosis of lung cancer. These
patients were then interviewed concerning
smoking habits, as were controls selected from
patients with other disorders (primarily non-
malignant) who were hospitalized in the same
hospitals at the same time.

Data for the cohort study were obtained from the
population of all physicians listed in the British
Medical Register who resided in England and
Wales as of October 1951. Information about
present and past smoking habits was obtained by
questionnaire. Information about lung cancer
came from death certificates and other mortality
data recorded during ensuing years.


Question 1: What makes the first study a case-control study?
Answer: This study compares people with lung cancer with the people without Lung Cancer to
examine differences in smoking.
As per definition of case-control study, it is the one that compares individuals who have a disease
with individuals who do not have a disease in order to examine the differences in exposure of risk
factors.




Question 2: What makes the second study a cohort study?
Answer: A type of study that collects data and follows a group of individuals who have received a
specific exposure, is classified as cohort study. The incidence of outcome of interest is traced over
time.The incidence in the exposed group is compared with the incidence in the group that is not
exposed.





The remainder of Part I deals with the case-control study.

Question 3: Why might hospitals have been chosen as the setting for this study?
Answer: The advantages for selecting hospitals for setting up this study is that, it is easier for
researcher to find patients as the hospitals are a pool already of people with or without Lung Cancer.
CDC / EIS Summer Course 2003: Smoking and Lung Ca - Student's Guide Page 3

Question 4: What other sources of cases and controls might have been used?
Answer: Other sources of cases and controls that might have been used are as follows:
1) Cancer Centres
2) Doctor Offices
3) Hospice Facilities

Question 5: What are the advantages of selecting controls from the same hospitals as cases?
Answer: Advantages of selecting controls from the same hospitals as cases are as follows:
1) Ease of data collection
2) Similar geographical locations between control cases.
3) More consistent between the two groups










Question 6: How representative of all persons with lung cancer are hospitalized patients with lung
cancer?
Answer: There are many people with lung cancer not receiving hospital care. Thus, hospitalized
patients with lung cancer are a small representation of all people with lung cancer.



Question 7: How representative of the general population without lung cancer are hospitalized
patients without lung cancer?
Answer: The sample of people without lung cancer at hospitals is much smaller than general
population without lung cancer. Also, it does not account for people without lung cancer that
are otherwise not sick.




Question 8: How may these representativeness issues affect interpretation of the study's results?
Answer: With small representativeness of this study, it would limit generalization and would certainly
suggest further research to be conducted to confirm the results.
CDC / EIS Summer Course 2003: Smoking and Lung Ca - Student's Guide Page 4

Over 1,700 patients with lung cancer, all under
age 75, were eligible for the case-control study.
About 15% of these persons were not interviewed
because of death, discharge, severity of illness, or
inability to speak English. An additional group of
patients were interviewed but later excluded when
initial lung cancer

diagnosis proved mistaken. The final study
group included 1,465 cases (1,357 males
and 108 females).

The following table shows the relationship
between cigarette smoking and lung
cancer among male cases and controls.


Table 1. Smoking status before onset of the present illness, lung cancer cases and matched controls
with other diseases, Great Britain, 1948-1952.

Cases Controls
Cigarette smoker 1,350 1,296
Non-smoker 7 61
Total 1,357 1,357


Question 9: From this table, calculate the proportion of cases and controls who smoked.

Proportion smoked, cases: (1350/1357) = 99.48% or, 99.5%

Proportion...