EPB PHC 6000 EPIDEMIOLOGY FALL, 1997

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Unit 3 Descriptive Epidemiology
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• 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.

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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. Demons
trate knowledge of cross-sectional surveys
conducted at the national level.
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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.
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Descriptive epidemiology
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.
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Person
Since disease not does occur at random What
kinds of people tend to develop a particular
disease, and who tends to be spared? Whats
unusual about those people?
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Person
Age the most fundamental factor to
consider when describing disease
occurrence. --- The incidence of most chronic
diseases increases with age. --- However,
the incidence of many infectious diseases is
highest in childhood. --- Some disorders show
bi-modal (two peak) distributions (i.e.
Hodgkins disease). This may reflect different
underlying etiologies.
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Person
Gender 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).
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Person
Race/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?
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Person
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.
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Discussion Question 1
What hypotheses might explain the highest
incidence of severe mental illness among the
lowest social classes?
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Discussion Question 1

• Social causation hypothesis membership (and
  factors) in low social classes produces
  schizophrenia and other mental illness.
• Social drift hypothesis mental disorders are
  disabling stigma and impaired income earning
  ability that occur with mental illness results in
  downward mobility.

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Place
Since 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
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Place
Infectious and chronic diseases show great
variation from one country to another. Some
differences may be attributed to --- Climate
--- Cultural factors --- Diet --- Genetics

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Place
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

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Place
Some differences in disease occurrence between
urban and rural locations may be attributed
to --- Diet --- Physical activity --- Hous
ing conditions (i.e. lead paint) --- Crowding
(i.e. spread of infection) --- Pollution

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Place
Some localized differences in disease
occurrence may be attributed to --- Carcinogen
ic exposure (i.e. radon) --- Geologic
formations (i.e. water hardness) --- Lifestyl
e

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Discussion Question 2
Regarding cross-country variation in disease
occurrence, what is a likely impact of migrating
from ones native land to a geographically and
culturally different location?
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Discussion 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.
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Time
Since 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
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Time
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.
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Time
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.
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Time
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.
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Discussion 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?
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Discussion Question 3

• Perhaps
• Daily hormonal fluctuations
• Conditioned responses (i.e. stress associated
  with return to work on Monday)

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Cohort Effects
Cohort 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.
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Clustering
Clustering An unusual aggregation of health
events grouped together in space or time.
i.e. Adverse reactions to vaccines Outbreak
of legionnaires disease in 1970s Early
1980s high number of cases of Kaposis sarcoma
in young homosexual men
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Clustering
Clustering Be careful where to identify a
cluster because of chance variation.