This document discusses cross-sectional studies, which measure exposure and health outcomes at the same point in time. It notes that cross-sectional studies can be descriptive, providing prevalence rates, or analytic, examining associations between exposures and outcomes. While able to generate hypotheses, cross-sectional studies cannot determine causation due to their inability to assess temporal relationships. The document also briefly touches on case reports and case series, which lack control groups for formally assessing relationships.

1. Cross-Sectional Studies
Dr. Win Aye Hlaing
Lecturer
Department of Epidemiology
University of Public Health, Yangon
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2. Types of Descriptive Study
• Cross-sectional (Prevalence) Study
• Case Report
• Case Series
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3. Types of Analytic Study
• Cross-sectional Analytic Study
• Cohort
• Case-Control
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4. Cross Sectional Studies
• possible relationship of increased serum cholesterol level
(the exposure) to evidence of CHD (the disease)
• in cross-sectional study, both exposure and disease
outcome are determined simultaneously for each subject
• viewing a snapshot of the population at a certain point in
time
• to imagine that we have sliced through the population,
capturing levels of cholesterol and evidence of CHD at the
same time
• we identify are prevalent cases of the disease
• but we do not know their duration
• also called a prevalence study
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5. • Cross-sectional study is a study in which all
the measurements are taken at a particular
point in time
• When a study only measures health outcome,
it is known as descriptive cross-sectional study
• When a study only measures both exposure
and health outcome at the same time, it is
known as analytic (comparative) cross-
sectional study
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7. Descriptive Cross-Sectional Study
• They provide a prevalence rate of a disease at a
point in time (point prevalence) or over a period
of time (period prevalence)
• The total population is the denominator for these
prevalence rates
• The study describes prevalence rate by personal
characteristics (e.g., age & sex), geographical
areas and time frame
• If cross-sectional studies are done serially over a
certain period, they can provide time trend of
disease prevalence
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8. Analytic (Comparative) Cross-Sectional Study
• A cross-sectional study is an observational study
in which exposure and disease are determined at
the same point in time in a given population
• Often deal with exposures that can not change,
such as blood type or other invariable personal
characteristics
• Cases in a cross-sectional study will over
represent cases with a long duration of illness
and under represent those with a short duration
of illness
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10. • association between increased cholesterol
level and CHD, we are left with several
problems
• First, we are identifying prevalent cases of
CHD rather than incident (new) cases
• may not be representative of all cases of CHD
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11. • Second, the presence or absence of both
exposure and disease was determined at the
same time in each subject
• it is not possible to establish a temporal
relationship
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12. Cross-Sectional Studies : Example
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13. Uses of Cross-sectional Studies
• Assessment of Health Status of a Population
• Trend Analysis (by serial cross-sectional surveys)
– Secular trend, Cyclic changes, Seasonal variation,
Epidemics
• Health Care Planning
– prioritization, target population & resource allocation
• Clues about Disease Causation & Prevention
– Hypothesis generation
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14. Cross-sectional Study Strengths
• Relatively feasible and not too time-consuming,
since there is no follow-up period (though
random sampling in a large population can be
expensive and problematic)
• We can study several diseases and/or exposures;
thus, it is useful for screening new hypotheses
• We can describe disease frequency and health
needs of a large population; thus, it is useful for
health planning
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15. Cross-sectional Study Weaknesses
• Potential temporal ambiguity (exposure and
disease)
• It is inefficient for studying rare or highly fatal
diseases or diseases with short durations of
expression
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16. Case reports and case series
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19. • Advantages of Case Series
– Useful in hypothesis formation, describing "clinical
experience“
– Easy and inexpensive to do in hospital settings
– Can constitute the case group for a case-control
study
• Disadvantages of Case Series
– With no comparison group, no formal assessment
of relationship b/t exposure and outcome
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