1. Some epidemiological principles and methods
Oye Gureje
Department of Psychiatry
University of Ibadan, Nigeria

2. What is epidemiology
Epidemiology is “the study of the distribution and determinants of health-related states or events in specified populations and the application of this study to the control of health problems” (Last et al., 1995).

3. Uses of epidemiology
Establishing the dimensions of morbidity and mortality as a function of person, place and time
Quantifying risks of developing morbidity
Identifying and defining syndromes
Describing the full clinical spectrum of disease and illness
Describing the natural history of disease
Identifying factors which influence or predict clinical course
Identifying causes of disease, disability and mortality
Evaluating methods of disease prevention and control
Morris JN. Uses of Epidemiology, 3rd Edition. Edinburgh: Churchill Livingstone, 1975

4. Study designs How is it done? On whom is it done? When is it done?
Observational: relying on naturally occurring variation in risk factors
Experimental: introducing exposure to protective factors or treatments by design
On whom is it done?
Individual: the person is the unit of observation
Ecological: a social group, community, state or nation is the unit of analysis
When is it done?
Cross-sectional (or retrospective) data collection
Case control study
Longitudinal data collection.
Permits the study of aging effect, cohort effect, time effect
Cohort (prospective) study

5. Scope of epidemiology Types: Settings Descriptive Analytic
Experimental
Settings
Community
Clinical

6. Community studies Descriptive Analytic Experimental Prevalence studies
Nosological questions: case definition: How is anxiety disorder different from normal anxious feelings?
Familial aggregation
Analytic
Risk factors
Prenatal exposure to famine and risk of schizophrenia in offspring
Genetic epidemiology
Experimental
Can be built on social or natural changes that create opportunities to study associated mental health consequences

7. Clinical epidemiology
Clinical epidemiology has been defined as the application of epidemiologic principles and methods to problems encountered in clinical medicine

8. Clinical studies Descriptive Analytic Experimental Outcome studies
E.g. value of maintenance treatment
Analytic
Cost effectiveness studies
Experimental
Clinical trials
Preventive trials

9. What is prevalence
Prevalence is the proportion of a population with a characteristic or disease at a particular time (point prevalence) or over a particular period (period prevalence)
It is the total number of individuals who have the disorder at a particular time (or during a particular period) divided by the population at risk of having the disorder at that given time/period.

10. To determine prevalence…..
The population at risk has to be known
The disease must be discrete and definable
The point in time or period has to be specified

11. Incidence
Incidence is the proportion of a population with new occurrence of a characteristic or disorder over a period of time

12. To determine incidence…..
The population at risk must be known
The characteristic or disorder has to be identifiable: must be amenable to discrete categorization
Must be possible to identify new from pre-existing cases with characteristic or disorder

13. Relationship between prevalence and incidence
Prevalence = incidence X duration

14. Defining the problem...
What is the condition: dimensional vs. categorical description
What is a “case”?
How is it to be defined?
Which ascertainment tool?
What classificatory system(s)
The multi-trait, multi-method approach

15. Defining the population...
What is the population at risk?
Is sampling necessary?
What is the sampling method?
Simple or complex

16. What type of survey? Single or two-stage?
Is a “pilot study” necessary?

17. The two-stage survey
First stage: a screen is applied as a case-finding tool
Second stage: a proportion of the screened population is comprehensively assessed (e.g. by interview)

18. Advantages of screening
A large population can be surveyed
A manageable sample can be assessed in detail
Subjects more likely to have disorder of interest can be selected for interview
More economical with resources

19. Screening tools Generic Specific General Health Questionnaire K10
Hopkins Symptom Checklist
Specific
Beck’s Depression Inventory
Centre for Epidemiologic Studies-Depression Scale
Geriatric Depression Scale
Hospital Anxiety and Depression Scale
Mini-Mental Stage Examination

20. Properties of the ascertainment tools
The properties of the screen
sensitivity
specificity
“cut-off” threshold
reliability
validity
The properties of the interview tool

21. Interview schedules Semistructured Fully structured
Present State Examination
Schedules for the Clinical Assessment in Neuropsychiatry (SCAN)
Schedule for the Assessment of Depression and Schizophrenia
Kiddie-SADS
Structured Clinical Interview for DSM
Fully structured
Composite Diagnostic International Interview

22. Other scales Rating scales Hamilton Depression Rating Scale
Brief Psychiatric Rating Scale
Positive And Negative Symptom Scale (PANSS)
WHO Disability Assessment Scale (WHO-DAS)
WHO Quality of Life Scale

23. Fully-structured interviews
Advantages
Can be administered by lay interviewers
Permit survey of large samples
Cost efficient
Challenges
Validity of measures
Eliciting interest and honest responses of respondents

24. Other epidemiologic methods
Case-control studies
Cohort studies
Clinical trials

25. The nature of case-control studies
Observational analytic epidemiologic study
Subjects are selected on the basis of an outcome:
Those who have it are “cases”
Those who do not have it are “controls”

26. Case-Control Studies: the rationale
Comparison of a group with condition of interest (“cases”) with a group without such condition (“controls”)
Designed to test hypothesis or explore factors/features associated with condition of interest
Look back at exposure rates

27. Case-control study
A cost-efficient method of studying the association of a putative risk factor with disease onset
Elements:
A sample of recent onset cases of the condition of interest
A sample of controls selected in a way that does not introduce sampling bias with respect to risk factors relative to cases
Measurements made on cases and controls using the same methods.

28. Case control vs cohort studies
SUBSJECT SELECTION
CASENESS
EXPOSURE
Perspective
Retrospective
Prospective
Sources of bias
Selection bias
Information bias
Outcome information only
Non-response bias
Resources
Quick
Relatively cheap
Lengthy
Relatively expensive
Useful for:
Rare outcomes
Single outcomes
Multiple exposures
Rare exposures
Single exposures
Multiple outcomes
Measure
Odds ratio
Relative risk

29. So…
Epidemiology is not just about prevalence and incidence, but also useful for decisions on:
Diagnosis
Treatment
Prevention