Upload
cathleen-henderson
View
216
Download
2
Embed Size (px)
Citation preview
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