1. Descriptive Epidemiology
Principles of Epidemiology
Lecture 6
Dona Schneider, PhD, MPH, FACE

2. 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

3. 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

4. Forecast of Cancer Deaths
Forecast of cancer deaths if present trends continue (Data from the American Cancer Society)

5. Cancer Death Rates by Site, United States, 1930-87
Figure 5-1. Cancer death rates by site, United States, Source: American Cancer Society (1991).

6. Possible Reasons for Changes in Trends
Artifactual
Errors in numerator due to
Changes 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

7. Death Rates for Individuals with Diabetes
Figure Drop in death rates for diabetes among 55 to 64 year old men and women, United States, , 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.)

9. ICD-10 International Classification of Disease (ICD) 10th Revision
ICD-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!

10. 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

11. 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

12. 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

13. Infant Mortality Rates by Race
Figure 3-3 Infant mortality rates by race: United States, 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.

14. 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

15. Cross-Sectional Studies
Cross 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

16. Factors Influencing Prevalence
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
Increased 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)

17. Prevalence of Congenital Malformations Across Maternal Age
Prevalence of congenital malformations across maternal age: California, Source: Reprinted from Croen, L.A., and Shaw, G.M., Young Maternal Age and Congenital Malformations: A Population-Based Study, American Journal of Public Health, Vol. 85, p. 711, with permission of the American Public Health Association, (C) 1995

18. Prevalence of Cigarette Smoking Among Successive Birth Cohorts
Changes in prevalence of cigarette smoking among successive birth cohorts of US men, Source: Reprinted from Strategies To Control Tobacco Use in the United States: A Blueprint for Public Health Action in the 1990s. p. 82, National Cancer Institute, National Institutes of Health, Publication No , 1992.

19. Comparing Cross-Sectional and Longitudinal Data
How you organize your data depends on your research question.
Cross Sectional Data
Cohort or
Longitudinal
Data 45 A B C D E 40 F 35 G 30 H 25 I
1955
1960
1965
1970
1975
Year of Examination
30 Year Olds in Successive Years

20. 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

21. 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

22. Breast Cancer Mortality and Dietary Fat Intake
Ecologic correlation of breast cancer mortality and dietary fat intake. Source: Reprinted from Carroll, K.K., Experimental Evidence of Dietary Factors and Hormone-Dependent Cancers, Cancer Research, Vol. 35, p.3379, with permission of Waverly Press, Inc., © 1975.