DESCRIPTIVE STUDIES Ahmed Mandil Prof of Epidemiology KSU

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DESCRIPTIVE STUDIES Ahmed Mandil Prof of Epidemiology KSU College of Medicine

DESCRIPTIVE STUDIES Ahmed Mandil Prof of Epidemiology KSU College of Medicine

Headlines • • • Introduction General concepts Descriptive versus analytical epidemiology Types of study

Headlines • • • Introduction General concepts Descriptive versus analytical epidemiology Types of study designs in health research Cross-sectional studies Measures

X &Y notations • X = independent variable (exposures, causes, risk factors) • Smoking,

X &Y notations • X = independent variable (exposures, causes, risk factors) • Smoking, obesity, asbestos, environmental factor, etc • Y = dependent variable (outcomes, effects, diseases, injuries, disabilities, complications, mortalities) • Respiratory systems cancers (larynx, trachea, lung, etc); diabetes; asbestosis (types of pneumoconiosis), etc

Types of Quantitative Research Designs – Descriptive X? Y? • What is X, Y?

Types of Quantitative Research Designs – Descriptive X? Y? • What is X, Y? – Correlational • Is there a relationship between X and Y? – Analytical ΔX ΔY? • Does a change in X cause a change in Y?

Are exposure and outcome linked (association, causation) ? Exposure Outcome

Are exposure and outcome linked (association, causation) ? Exposure Outcome

Basic Questions in Health Research To link exposure and outcome, we ask: – What

Basic Questions in Health Research To link exposure and outcome, we ask: – What is the exposure? – Who are the exposed? – What are the potential outcomes ? – What approach will we take to study the relationship between exposure and outcome ?

Data Collection Methods • Primary: where the investigator is the first to collect the

Data Collection Methods • Primary: where the investigator is the first to collect the data. Sources include: medical examinations, interviews, observations, etc. Merits: less measurement error, suits objectives of the study better. Disadvantage: costly, may not be feasible. • Secondary: where the data is collected by OTHERS, for other purposes that those of the current study. Sources include: individual records (medical / employment); group records (census data, vital statistics)

Descriptive versus Analytical Epidemiology DESCRIPTIVE EPIDEMIOLOGY ANALYTIC EPIDEMIOLOGY Examining the distribution of a disease

Descriptive versus Analytical Epidemiology DESCRIPTIVE EPIDEMIOLOGY ANALYTIC EPIDEMIOLOGY Examining the distribution of a disease in a population, and observing the basic features of its distribution in terms of time, place, and person. Typical study design: community health survey (approximate synonyms cross-sectional study, descriptive study) Triad: person, place, time Testing a specific hypothesis about the relationship of a disease to a putative cause, by conducting an epidemiologic study that relates the exposure of interest to the disease of interest. Typical study designs: cohort case-control Triad: agent, host, environment

Study Designs in Health Research • Qualitative – Focus group discussions – Key informant

Study Designs in Health Research • Qualitative – Focus group discussions – Key informant studies – Ethnographic studies – Bibliographic studies – Others • Quantitative – Observational • Descriptive, e. g. crosssectional (surveys) • Analytical, e. g. casecontrol; cohort – Experimental (also analytical) • Randomized Clinical Trials • Community Intervention Studies

Observational Studies • Non-experimental • Observational because there is no investigator(s) intervention • Exposures

Observational Studies • Non-experimental • Observational because there is no investigator(s) intervention • Exposures occur in a “non-controlled” environment • Individuals can be observed currently (snap shot), prospectively, retrospectively, or a combination

Observational Designs (3 Broad Types) • Exploratory: used when the state of knowledge about

Observational Designs (3 Broad Types) • Exploratory: used when the state of knowledge about the phenomenon is poor: small scale; of limited duration. • Descriptive: used to formulate a certain hypothesis: small / large scale. Examples: casestudies; cross-sectional studies • Analytical: used to test hypotheses: small / large scale. Examples: case-control, cross-sectional, cohort.

Cross-sectional studies An “observational” design that surveys exposures and outcomes at a specified point

Cross-sectional studies An “observational” design that surveys exposures and outcomes at a specified point in time (a cross-section of the population) time Study only exists at this point in time

Cross-Sectional Design exposure present, outcome present Study population exposure absent, outcome present exposure present,

Cross-Sectional Design exposure present, outcome present Study population exposure absent, outcome present exposure present, outcome absent exposure absent, outcome absent time Study only exists at this point in time

Cross-sectional Studies • Often used to study conditions that are relatively frequent with long

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-based surveys • Not suitable for studying rare or highly fatal diseases or a disease with short duration of expression

Cross-sectional Studies: Advantages • Feasible; quick; economic; allows study of several diseases / exposures;

Cross-sectional Studies: Advantages • Feasible; quick; economic; allows study of several diseases / exposures; useful for estimation of the population burden, health planning and priority setting of health problems

Cross-sectional studies: Disadvantages • Weakest observational designs (measure prevalence, not incidence of disease) •

Cross-sectional studies: Disadvantages • Weakest observational designs (measure prevalence, not incidence of disease) • The temporal sequence of exposure and outcome may be difficult or impossible to determine (temporal ambiguity) • Usually don’t know when outcome (e. g. disease) occurred • Not suitable for study of rare events and quickly emerging diseases

Measures (I) • Frequency measures (for estimation of the magnitude of the problem /

Measures (I) • Frequency measures (for estimation of the magnitude of the problem / outcome): incidence (cohort studies), prevalence (cross-sectional studies) • Effect measures (for quantification of the magnitude of association between the exposure and outcome): risk ratio or relative risk (cohort studies); odds ratio (estimates the RR in both cross-sectional and case control studies)

Measures (II) Impact fractions (to estimate the proportion of the outcome attributed to the

Measures (II) Impact fractions (to estimate the proportion of the outcome attributed to the exposure): – Attributable fractions (for causal associations) – Prevented fractions (for preventive associations)

Headlines • • • Introduction General concepts Descriptive versus analytical epidemiology Types of study

Headlines • • • Introduction General concepts Descriptive versus analytical epidemiology Types of study designs in health research Cross-sectional studies Measures

References • Last J. A dictionary of epidemiology. 5 th edition. Oxford: Oxford University

References • Last J. A dictionary of epidemiology. 5 th edition. Oxford: Oxford University Press, 2008. • Holzemer WL. Advanced Quantitative Methods. University of California, San Franciso, USA. • Songer T. Study designs in epidemiological research. In: South Asian Cardiovascular Research Methodology Workshop. Aga-Khan and Pittsburgh universities. • Kelsey JL, Thompson WD, Evans AL. Methods in observational epidemiology. New York: Oxford University Press.