Measuring the Occurrence of Disease 1

Sue Lindsay, Ph.D., MSW, MPH

Division of Epidemiology and Biostatistics

Institute for Public Health

San Diego State University

Pandemic Preparedness

www.readysandiego.org

http://www.ready.gov

Types of Measurement Commonly Used in Epidemiology

• Proportions

• 25 males/50 people = 0.50 or 50%

• Rates

• number of cases/population, cohort, or sample

• a specified time period

• all in the denominator have the potential to be in the numerator

• Ratios

• An expression of the relationship between numerator and

denominator where they are two separate and distinct

quantities: 25 females/29 males

Common Sources of Data For Stages in the Natural History of Disease

Disease

Onset

Symptoms Seek

Care

Diagnosis/

Treatment

Healthy

Interviews

Medical RecordsSources

Of Data }

Outcome

Surveys

Birth Records

Death Records

Measures of Morbidity

• Point Prevalence

• Period Prevalence

• Incidence

• Cumulative incidence

• Incidence density

• Lifetime incidence

Types of Populations

• Dynamic Population

• Fixed Population

Point Prevalence

Prevalence

Per 1,000=

No. of Existing Cases of Disease

In a Population

No. of Persons in the Population

At a Specific Point in TimeExpressed as either a rate or percentage

Period Prevalence

Prevalence

Per 1,000=

No. of Existing Cases of Disease

In a Population

No. of Persons in the Population

During a Specified Period of Time

Expressed as a rate: 50/10,000 persons in City A have diabetes in 2007

Incidence

Incidence

Per 1,000=

No. of New Cases of Disease

In a Population

No. of Persons at Risk of

Developing Disease

During a Specified Period of Time

Expressed as a rate: 10 persons/10,000 developed new diabetes in 2007

Cumulative Incidence

No. of New Cases of Disease in a Time Period

Number of persons in the population

Expressed as a percentage:

2% of the 500,000 persons in City A developed diabetes in 2007

Lifetime Incidence(lifetime cumulative incidence)

No. of New Cases of Disease in a Time Period

Number of persons in the population

Expressed as a percentage:

20% of the persons in City A developed diabetes in their lifetimes

Fixed Populations

(study populations)

Incidence Density

No. of New Cases of Disease in a Time Period

Person-time at risk for the same time period

Calculation of Person-Time

Example:

10 people followed for 3 years =

30 person-years or 360 person-months

3 people followed for 10 years =

30 person-years or 360-person months

Example Calculations

0 1 2 3 4

1

2

3

4

5

6

7

8

Studysubjects

• Incidence rate = 5/8 = 62.5/100 in 3 years• Incidence density = 5/18 person-years at risk

= 5/18 = 27.8/100 person-years• Prevalence in year 2 = 3/6 = 50/100

Time (years)

Patient died

Patient lost to follow-up Red = disease

Green= no disease

Incidence: Important Concepts

• Incidence is a measure of the risk of disease!

• As incidence goes up, so does risk

• The numerator is only NEW cases of disease

• The denominator must include only those persons at risk for developing the disease

• Time unit must be clearly specified

• All persons at risk must be followed the entire time period

Uterine Cancer Incidence Rates, Alameda County

Years

Rat

e P

er 1

00,0

00

0

10

20

30

40

50

60

70

80

1960 1965 1970 1975

all women

only women withouthysterectomy

Differences BetweenIncidence and Prevalence

• Prevalence is only a snapshot of the population at a point or period in time - who has the disease and who does not

• Prevalence tells you nothing about how long patients have had the disease in the population

• Incidence includes only new cases or events among those at risk, and is a measure of risk of disease

Factors that Affect Prevalence:

• Prevalence rates are increased by:

• Immigration of cases or those with a high potential to become cases (elderly)

• Emigration of healthy individuals

• Increases in the duration of the disease

(earlier detection, better treatment, reduced fatality, etc.)

• Increases in incidence (occurrence of new cases)

Factors that Affect Prevalence:

• Prevalence rates are decreased by:• Immigration of healthy persons • Emigration of persons with disease• Improved cure rates• Increased death rates• Decreases in incidence (occurrence of new

cases)• Shorter duration of the disease

Examples of Point and Period Prevalence, Incidence, and Cumulative Incidence

• Do you currently have asthma?

• Have you had asthma during the last year?

• Did you have your first episode of asthma this year?

• Have you ever had asthma?

• Point Prevalence

• Period Prevalence

• Incidence

• Cumulative or Lifetime Incidence

The Relationship Between Incidence and Prevalence

Prevalence = Incidence x Duration

Low incidence and long duration

Incidence

Low Risk

N = 4/yr.

Duration

Long

25 Yrs

(25 X 4) Prevalence

100/1,000

High incidence and short duration

Incidence

High Risk

N = 20/yr.

Duration

Short

3 Yrs

(20 X 3) Prevalence

60/1,000

Prevalence of Coronary Heart Disease (CHD) Among 4,469 Persons, Framingham Study

0

5

10

15

20

25

30

35

40

45

50

30-44 45-62

Men

Women

Rat

e P

er 1

,000

Age

Incidence of Coronary Heart Disease (CHD) Among 4495 Persons,

Framingham Study

0

20

40

60

80

100

120

140

30-39 40-49 50-59

Men

WomenR

ate

Per

1,0

00

Age

General Sources of Morbidity Statistics

Disease

Reporting

Hospitals and

Clinics

Case-finding

Programs

Insurance

Absenteeism

Records

Military Service

Tax-Financed Medical Care

Physical Exams in Industry

Morbidity Surveys

Sources of Morbidity Information About Cancer

Medical records

Hospital Discharge Data

Billing Records

Medicare

Cancer registries

SEER (Surveillance, Epidemiology, and End Results)

SEER/Medicare linked data sets

HEDIS (Health Plan Employer Data and Information Set)

SEER

SEER – Surveillance, Epidemiology, and End Results

Summary of cancer registry data from 4 states and 5 large metropolitan areas

Represents 10% US population

Used to estimate cancer rates for the nation

HEDIS

Health Plan Employer Data and Information Set

Established by National Committee for Quality Assurance

Quality measures to compare health insurance plans

Incidence and Prevalence: Challenges with the Numerator

• Who has the disease?• Diagnostic criteria can strongly impact rates

• Can be differences in case definitions

• Case definitions may change over time

• There may be changes in the quality of screening services

• Who Should Be Counted? • Is case finding accurate and inclusive? How large is the

effort to find all cases?

• Case finding by interview may yield inaccurate counts

PREVALENCE OF DEMENTIA BY DIFFERING CRITERIA

0

5

10

15

20

25

30

DSM-III DSM-IIIR DSM-IV ICD-9 ICD-10 CAMDEX CLINCONS

CRITERIA

PR

EV

AL

EN

CE

%

Number of Cases of Rape

0

200,000

400,000

600,000

800,000

Nu

mb

er

of

Ra

pe

s

97,47097,470

260,300260,300

683,000683,000

US Dept. of JusticeBureau of Justice Statistics

(NCVS) 1995

National Women’s Study1990

FBI Uniform CrimeReport for 1995

AIDS CASES BY YEAR, 1984 - 1993

0

5

10

15

20

25

30

35

40

1984 1985 1986 1987 1988 1989 1990 1991 1992 1993

Years

Th

ou

san

ds

of

Cas

es

SOURCES OF ERROR IN INTERVIEW SURVEYS

• Patient:

• Unaware of Disease

• Misunderstanding of Diagnosis

• Inaccurate Recall

• Refusal to Respond

• Altering of Response

• Interviewer :

• Interviewer Bias

• Selection of Subjects

• Lack of standardization of interview format

Limitations of Hospital Data Sources

• Admissions are selective

• Hospital records are not designed for research

• The denominator or population at risk is usually not known

Incidence and Prevalence:Problems with the Denominator

• Selective Undercounting – Who doesn’t get counted?

• Census Geographic Boundary changes

• Changes in Defined Populations Over Time

Surveillance

A comprehensive system designed for the on-going detection and monitoring of the occurrence of health-related events or exposures in a target population

Key Features of Surveillance

• Continuous data collection and evaluation

• An identified target population

• A standard definition of the outcome of interest

• Emphasis on timeliness of collection and dissemination of results

• Use of data for purposes of disease investigation or control