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

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).

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

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

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

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

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

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

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.

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

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

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

Relationship between prevalence and incidence Prevalence = incidence X duration

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

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

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

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)

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

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

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

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

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

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

Other epidemiologic methods Case-control studies Cohort studies Clinical trials

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”

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

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.

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

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