Unit 3:

Descriptive Epidemiology

Unit 3 Learning Objectives:

1. Characterize the major dimensions of descriptive epidemiology: Person, Place, Time

2. Recognize how measurement and quantification of health outcomes by person, place, and time can assist in planning health services.

3. Recognize how measurement and quantification of health outcomes by person, place, and time can provide clues to etiology of health-related events.

4. Recognize the characteristics, strengths, and limitations of ecologic (aggregate) studies, case reports, and case series.

Unit 3 Learning Objectives (cont.):

5. Understand the “ecologic fallacy.”

6. Understand the concepts of cohort effects and clustering.

7. Understand the design features and information provided by cross-sectional surveys.

8. Recognize the strengths and limitations of cross-sectional surveys.

9. Demonstrate knowledge of cross-sectional surveys conducted at the national level.

Assigned Readings:

Textbook (Gordis):

Chapter 13, pages 204-206 (Ecologic studies)

Koepsell and Weiss: Person place, and time. In Epidemiologic Methods, Chapter 7, pages 147-178.

Purpose: To characterize the amount and distribution of disease within a population.

In other words …. To identify health problems and patterns of disease that exist.

Descriptive studies generally precede analytic studies designed to investigate determinants of disease.

Thus, descriptive studies often help to generate research hypotheses.

Descriptive epidemiology

PersonPersonSince disease not does occur at

random:

What kinds of people tend to develop a particular disease, and who tends to be spared? What’s unusual about those people?

PersonPerson

• Age – the most fundamental factor to considerwhen describing disease occurrence.

--- The incidence of most chronic diseases increases with age.

--- However, the incidence of many infectiousdiseases is highest in childhood.

--- Some disorders show bi-modal (two peak) distributions (i.e. Hodgkin’s disease). Thismay reflect different underlying etiologies.

PersonPersonGender – biological and non-biological factors related to gender may impact disease risk.

--- In all developed countries, life expectancy is higher in females and males – principally

due to lower heart disease mortality.

--- However, many chronic diseases occur more frequently in women (depression, lupus, etc.)

--- As lifestyles continue to become more similar, a question is whether mortality rates will become more similar (i.e. environment vs. biology).

PersonPersonRace/ethnicity – difficult to define, and to identify which characteristics may relate to disease occurrence. Remarkable variation exists in rates of disease occurrence across racial and ethnic groups.

--- Genetics?--- Socioeconomic status?--- Environmental exposures?--- Access to health care?--- Lifestyle factors?

PersonPerson

Social class – summarizing variable (SES), unreliably measured, that links:

--- Occupation--- Education--- Area of residence--- Income--- Lifestyle

Despite its unreliability, SES is consistently associated with mortality in a gradient fashion.

Discussion Question 1Discussion Question 1

What hypotheses might explain the

highest incidence of severe mental

illness among the lowest social classes?

Discussion Question 1Discussion Question 1

1. Social causation hypothesis: membership

(and factors) in low social classes

produces schizophrenia and other mental

illness.

2. Social drift hypothesis: mental disorders

are disabling – stigma and impaired income

earning ability that occur with mental

illness results in downward mobility.

PlacePlaceSince disease not does occur at

random:

Where is the disease especially common or rare, and what is different about those places?

Investigation by place includes:

• Across countries (international)• Within country variation• Urban/rural differences• Localized areas

• Infectious and chronic diseases show great variation from one country to another.

• Some differences may be attributed to:

--- Climate--- Cultural factors--- Diet--- Genetics

PlacePlace

• Infectious and chronic diseases also show considerable variation within a country(i.e. multiple sclerosis varies by latitude in the U.S.).

• Some differences may be attributed to:--- Climate--- Geology--- Latitude--- Environmental pollution--- Race/ethnicity

PlacePlace

• Some differences in disease occurrence between urban and rural locations maybe attributed to:

--- Diet--- Physical activity--- Housing conditions (i.e. lead paint)--- Crowding (i.e. spread of infection)--- Pollution

PlacePlace

• Some localized differences in disease occurrence may be attributed to:

--- Carcinogenic exposure (i.e. radon)--- Geologic formations (i.e. water

hardness)--- Lifestyle

PlacePlace

Discussion Question 2Discussion Question 2

Regarding cross-country variation in

disease occurrence, what is a likely

impact of migrating from one’s native

land to a geographically and culturally

different location?

Discussion Question 2Discussion Question 2

For many disorders, particularly chronic

diseases, migrants begin to assume

disease rates of the host country in just in

a few generations.

This provides strong evidence for the

influence of environmental factors since

genetics are relatively stable over time.

TimeTimeSince disease not does occur at

random:

How does disease frequency change over time, and what other factors are temporally associated with those changes?

The occurrence of health-relate events can vary by time:

• Secular trends• Cyclic fluctuations• Point epidemics

TimeTime

• Secular trends refer to gradual changes in disease occurrence over long periods of calendar time.

--- Example: In the U.S., mortality from heart disease has been gradually declining, whereas cancer mortality has been gradually increasing.

TimeTime

• Cyclic fluctuations refer to shorter-term increases and decreases in disease occurrence over a period of years, or within a year.

--- Fluctuations in respiratory infection deaths over a few

years--- Seasonal variation of

infections, heart attacks, etc.

TimeTime

• Point epidemic refers to increased disease occurrence among a group of people exposed almost simultaneously to an etiologic factor (i.e. pathogen, contaminant).

--- Despite exposure at a common

point in time, the actual time of disease onset may vary.

Discussion Question 3Discussion Question 3

In the U.S., heart attacks tend to occur

more frequently in the early morning

hours, and on Mondays.

What are some possible etiological factors

associated with this phenomenon?

Discussion Question 3Discussion Question 3

Perhaps:

1. Daily hormonal fluctuations

2. Conditioned responses (i.e. stress

associated with return to work on

Monday)

Cohort EffectsCohort EffectsCohort effect: Long-term variation in

disease occurrence among a group of persons who share something in common.

i.e.• Occupational exposures during a

specific time period.

• Birth year or era and changes in lifestyle characteristics such as smoking habits.

ClusteringClusteringClustering: An unusual aggregation of

health events grouped together in space or time.

i.e.• Adverse reactions to vaccines

• Outbreak of legionnaires’ disease in 1970’s

• Early 1980’s – high number of cases of Kaposi’s sarcoma in young homosexual men

ClusteringClusteringClustering: Be careful where to identify

a cluster because of chance variation.

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