Transcript EPB PHC 6000 EPIDEMIOLOGY FALL, 1997

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 crosssectional 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.
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.
Person
Since 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?
Person
Age – the most fundamental factor to consider
when describing disease occurrence.
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The incidence of most chronic diseases
increases with age.
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However, the incidence of many infectious
diseases is highest in childhood.
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Some disorders show bi-modal (two peak)
distributions (i.e. Hodgkin’s disease). This
may reflect different underlying etiologies.
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).
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?
Person
Social class – summarizing variable (SES),
unreliably measured, that links:
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Occupation
Education
Area of residence
Income
Lifestyle
Despite its unreliability, SES is consistently
associated with mortality in a gradient
fashion.
Discussion Question 1
What hypotheses might explain the
highest incidence of severe mental
illness among the lowest social classes?
Discussion 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.
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
Place
• Infectious and chronic diseases show
great variation from one country to
another.
• Some differences may be attributed to:
---------
Climate
Cultural factors
Diet
Genetics
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
Place
• Some differences in disease occurrence
between urban and rural locations may
be attributed to:
-----------
Diet
Physical activity
Housing conditions (i.e. lead paint)
Crowding (i.e. spread of infection)
Pollution
Place
• Some localized differences in disease
occurrence may be attributed to:
-------
Carcinogenic exposure (i.e. radon)
Geologic formations (i.e. water
hardness)
Lifestyle
Discussion 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 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.
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
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.
Time
• Cyclic fluctuations refer to shorterterm 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.
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.
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?
Discussion Question 3
Perhaps:
1. Daily hormonal fluctuations
2. Conditioned responses (i.e. stress
associated with return to work on
Monday)
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.
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
1970’s
• Early 1980’s – high number of cases of
Kaposi’s sarcoma in young homosexual
men
Clustering
Clustering: Be careful where to identify
a cluster because of chance variation.
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