Download - Descriptive Epidemiology Principles of Epidemiology Lecture 6 Dona Schneider, PhD, MPH, FACE
Descriptive Epidemiology
Principles of Epidemiology
Lecture 6
Dona Schneider, PhD, MPH, FACE
Epidemiology (Schneider)
Objectives of Descriptive Epidemiology To evaluate trends in health and disease and
allow comparisons among countries and subgroups within countries
To provide a basis for planning, provision and evaluation of services
To identify problems to be studied by analytic methods and to test hypotheses related to those problems
Epidemiology (Schneider)
Descriptive Studies Relatively inexpensive and less time-consuming than analytic
studies, they describe Who gets sick and/or who does not Where rates are highest and lowest Temporal patterns of disease
Seasonality Secular trends which are affected by
Changes in diagnostic techniques Changes in the accuracy of the denominator data Changes in the age distribution of the population Changes in survival from improved treatment or disease mutation Changes in actual disease incidence
Forecast of Cancer Deaths
41 65 85118
158211
268311
382443
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100
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1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Years
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Forecast of cancer deaths if present trends continue (Data from the American Cancer Society)
Cancer Death Rates by Site, United States, 1930-87
Figure 5-1. Cancer death rates by site, United States, 1930-1987. Source: American Cancer Society (1991).
Epidemiology (Schneider)
Possible Reasons for Changes in Trends Artifactual
Errors in numerator due toChanges in the recognition of disease
Changes in the rules and procedures for classification of causes of death
Changes in the classification code of causes of death
Changes in accuracy of reporting age at death
Errors in the denominator due to error in the enumeration of the population
Death Rates for Individuals with Diabetes
Figure 3-27. Drop in death rates for diabetes among 55 to 64 year old men and women, United States, 1930-1960, due to changes in ICD coding. (From US Public Health Service publication no. 1000, series 3, No. 1. Washington, DC, US Government Printing Office, 1964.)
Epidemiology (Schneider)
ICD-10 International Classification of Disease (ICD) 10th
RevisionICD-10 has 8,000 categories vs. only 4,000 for ICD-9
ICD-10 uses 4 digit alphanumeric system where ICD-9 uses 4 digit numeric system only (much more detail available with ICD-10)
Rules for coding simplified
Will create discontinuities!
Epidemiology (Schneider)
ICD-10 (cont.) Notable improvements in the content and format
of ICD-10 include: The addition of information relevant to
ambulatory and managed care encounters Expanded injury codes The creation of combination diagnosis/symptoms
codes to reduce the number of codes needed to fully describe a condition
Greater specificity in code assignment
Epidemiology (Schneider)
ICD-10 (cont.)
At present ICD-10 is widely used in Europe
In the US, however, migration to ICD-10 is complicated by the fact that ICD-9-CM is embedded in hospital billing systems NCHS developed a timeline to have ICD-10-
CM in use for morbidity diagnoses by 2001
Epidemiology (Schneider)
Possible Reasons for Changes in Trends (cont.)
Real Changes in age distribution of the population
Changes in survivorship
Changes in incidence of disease resulting from Genetic factors
Environmental factors
Figure 3-3 Infant mortality rates by race: United States, 1950-1991. Source: Reprinted from National Center for Health Statistics, Advance Report of Final Mortality Statistics, 1991, Monthly Vital Statistics Report, Vol. 42, No. 2, p. 11, 1993.
Infant Mortality Rates by Race
Epidemiology (Schneider)
Case Reports Case reports (case series) – report of a single
individual or a group of individuals with the same diagnosis
Advantages You can aggregate cases from disparate sources to
generate hypotheses and describe new syndromes
Example: hepatitis, AIDS, “pool fingers”
Limitations You cannot test for statistical association because there is
no relevant comparison group
Cross-Sectional StudiesCross sectional studies or prevalence studies measure disease and exposure
simultaneously in a well-defined population Advantages
Prevalence studies cut across the general population, not simply those seeking medical care
They are good for identifying the prevalence of common outcomes, such as arthritis, blood pressure or allergies
Limitations You cannot determine whether exposure preceded disease Since you determine prevalent rather than incident cases, results will be
influenced by survival factors
Remember: P = I x D
Factors Influencing PrevalenceIncreased by:
Longer duration of the disease
Prolongation of life of patients without cure
Increase in new cases
(increase in incidence)
In-migration of cases
Out-migration of healthy people
In-migration of susceptible people
Improved diagnostic facilities
(better reporting)
Decreased by:Shorter duration of
disease
High case-fatality rate from disease
Decrease in new cases (decrease in
incidence)
In-migration of healthy people
Out-migration of cases
Improved cure rate of cases
Prevalence of Congenital Malformations Across Maternal Age
Prevalence of Cigarette Smoking Among Successive Birth Cohorts
Comparing Cross-Sectional and Longitudinal Data
How you organize your data depends on your research question.
45 A B C D E
40 B C D E F
35 C D E F G
30 D E F G H
25 E F G H I
1955 1960 1965 1970 1975
Year of Examination
Cohort or
Longitudinal
Data
30 Year Olds in Successive
Years
Cross Sectional Data
Epidemiology (Schneider)
Correlational Studies Correlational studies (ecological studies) use
measures that represent characteristics of entire populations (areal aggregates) to describe outcomes in relation to some factor of interest such as age, time, utilization of services, or exposures
ADVANTAGES You can generate hypotheses for case-control studies
and environmental studies You can target high-risk populations, time-periods, or
geographic regions for future studies
Epidemiology (Schneider)
Correlational Studies (cont.) LIMITATIONS
Because data are for groups, you cannot link disease and exposure in individuals Example: Percentage of teenagers taking drivers education and
fatal teenage car accidents study done by National Safety Council
You cannot control for potential confounders Data represent average exposures rather than individual
exposures, so you cannot determine a dose-response relationship
Caution must be taken to avoid drawing inappropriate conclusions, or ecological fallacy
Breast Cancer Mortality and Dietary Fat Intake