Ora Paltiel, MD, MSc

Braun School of Public Health & Community Medicine

Hebrew University of Jerusalem Hadassah Medical Organization

Israel

Epidemiological Reasoning

Using Cancer Statistics

Or, how to use descriptive statistics to raise hypotheses

Issues to be discussed

Validity of data• Reporting• Confounding • Effect

modification

Using Descriptive Data

• Burden of Disease• Planning• Hypothesis raising• Measuring

progress

What are the objectives of epidemiology?

1. To determine the extent of disease (states of health) and/or behaviors in the community.

2. To identify the etiology or the cause/s of a disease and the risk factors - that is, factors that increase a person’s risk for a disease.

3. To study the natural history and prognosis of disease.

4. To evaluate new preventive and therapeutic measures and new modes of health care delivery.

5. To provide the foundation for developing public policy and regulatory decisions relating to public health problems.

Objectives of epidemiology

“When we measure, we know better”

- Center for Disease Control (CDC), Atlanta, Georgia,USA

The epidemiological

tool-box

Kaposi sarcoma in New York

0

20

40

60

80

100

73 74 75 76 77 78 79 80 81 82

Year

No

. of

case

s

The context of disease reporting

Lowest cancer death rate

In the Former Yugoslav Republic of Macedonia, only 6

people per 100,000 of population die from cancer

each year

Lifetime risk of developing breast cancer, 1940-1987

0

2

4

6

8

10

12

1940 1950 1960 1970 1980 1987

perc

ent

of w

omen

YEARONE IN.…

194020

195015

196014

197013

198011

1987 9

Source: American Cancer Society, 1991

Lifetime risk of developing breast cancer, 1940-1987 cont’d

Descriptive epidemiology - hypothesis raising rarely provides enough evidence for causation

Person: characteristics for study include:

• Age

• Gender

• Religion

• Marital status

• Ethnicity

• Occupation

• Socio-economic class

• Heredity vs. Environment

Age-specific rates of Breast Cancer Mortality

Russian Federation

Israel

Population Pyramids 1998

Trends of Cervical Cancer Mortality in Europe and North

America

Age-standardized cervical cancer death rates (and 95% confidence intervals) per

100 000 women in urban Canada by neighbourhood income quintile from 1971

to 1996. Q1 = richest Q5 = poorest .

Place and time

Time trends - raise hypotheses regarding environmental factors or results of medical care

Geographic variation - on small + large scale, environmental genetic factors

Study of migrants: important for separating environmental from genetic factors

Numbers of cases of cancer at 16 anatomical sites in developed and in developing countries, with relative ranks

Lung Cancer Mortality for Women 1998, ASR/100000

Lung Cancer Mortality for men 1998, ASR/100000

Age-adjusted cancer death rates, males by site, US, 1930-1996

Age-adjusted cancer death rates, females by site, US, 1930-1996

Estimated annual percent changes in mortality from all types of cancer in the US over 2 periods 1973-

1990 and 1991-1995, according to age group

0.7

0.3

0

-1.8

-2.1

-1.7

-1.3

-2.6

-3.4

-0.6

1.1

0.9

0.9

-0.9

-1.4

-1.1

-2.2

-3

0.4

0.4

-4 -2 0 2 4

>85

75-84

65-74

55-64

45-54

35-44

25-34

15-24

<15

All ages

Ag

e g

rou

p (

yr)

Annual change (%)

1973-1990

1991-1995

Japanese colon cancer incidence:

Japan Hawaii California

- rate is affected by age at immigration

- for breast cancer: 2 generations required for rate

Place and time cont’d

Low

Intermediate

High

Biases in migrant studies

1) Different reporting

2) Different diagnostic criteria

3) Migrants are selected group

Clinical observati

on

Descriptive data

Hypothesis raising

Where does evidence come from?

Clinical observati

on

Descriptive data

Hypothesis raising

Analytical studies

Hypothesis testing