This document provides an introduction to epidemiological studies. It defines epidemiology as the study of disease distributions in populations and factors that influence distribution. It describes the hierarchy of evidence in epidemiology, including descriptive and analytical studies. Descriptive studies like case reports and prevalence surveys describe disease patterns, while analytical studies test hypotheses. The main analytical study designs are cross-sectional studies, case-control studies, and prospective cohort studies. Each design differs in sampling, time orientation, and ability to infer causality. Cross-sectional studies measure prevalence, case-control studies infer causality retrospectively, and prospective cohort studies follow exposed groups over time to measure incidence.
7. • non-experimental
• there is no individual intervention
• individuals can be observed
prospectively, retrospectively, or
currently
Observational Studies
8. • Routine data
• Health and productivity profile
• Case studies/reports
• Case series
• Field surveys (ex KAP)
• Prevalence surveys
Descriptive Studies
9. Routine data
• Examination of data routinely
collected and submitted by
various animal facilities
– Slaughterhouses
– laboratories
Health and productivity profile
11. Field Survey
• Locally known as KAP survey-
“KAP” for knowledge,
attitudes and practices
12. Comparative studies testing a hypothesis
cross-sectional
(a snapshot; no idea on cause-and-effect
relationship)
cohort
(prospective; cause-and-effect relationship ca
inferred)
case-control
(retrospective; cause-and-effect relationship
inferred)
Analytical Studies
14. Cross-sectional studies
• Selection of a random sample of animals from
a population and the disease status and
exposure status (potential risk factors) are
measured simultaneously
• To determine magnitude of disease in each
exposure group
16. Cross-sectional studies
• Often used to study conditions that are
relatively frequent with long duration of
expression (nonfatal, chronic conditions)
• It measures prevalence, not incidence of
disease
• Example: community surveys
17. Case-control studies
• Synonym: case-referent study
• study of persons/animals with the disease (or
another outcome variable) of interest and a
suitable control group of persons/animals
without the disease (comparison group)
25. Criteria Cross- sectional Case-control P. Cohort
Sampling Random sample
of the study
population
separate
samples of
diseased and
non-diseased
units
separate
samples of
exposed and
non-exposed
Unit
Time One time Usually
retrospective
Follow-up over a
specified period
Risk Prevalence none Incidence density
and cummulative
incidence
Comparison of
risk
Odds ratio
Relative risk
Odds ratio Odds ratio
Relative risk
26. Cross-sectional
Study
Case-control
Study
Cohort study
Measure of
Disease frequency
Prevalence
Prevalence Incidence
Direction of
Investigation
Momentary/one point
Retrospective
Retrospective Prospective (follow up
over specified period)
Samples
(selections)
1 sample from the
Population
1 group of cases,
1 group of controls
1 cohort of exposed,
1 cohort of unexposed
Primary measure
Of association
Involved
Prevalence odds ratio Odds ratio
Relative risk;
Attributable risk
Marginal
Conditions
Quick
Relatively cheap
Quick
Relatively cheap
Time-consuming
Relatively costly
Applicability
Permanent risk factors
Quite common diseases
More general
Rare diseases More general
Data quality As good as diagnosis Errors in historic data
As good as diagnosis
Sample sizes Large (low prevalence) Relatively small
Large (dropout, low
Incidence)
No causal evidence but
association between
disease and risk factor.
Limited causal evidence
Causal evidence through
evidence of temporality
34. Field trial
A field trial is
• a comparative study involving new treatments
or preventive measures applied under natural,
field or semi-field conditions.
