1. DESCRIPTIVE STUDIES Ahmed Mandil Prof of Epidemiology KSU College of Medicine

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

3. 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

4. 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?

5. Are exposure and outcome linked (association, causation) ?

6. 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 ?

7. 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)

8. 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

9. Study Designs in Health Research
Qualitative
Focus group discussions
Key informant studies
Ethnographic studies
Bibliographic studies
Others
Quantitative
Observational
Descriptive, e.g. cross-sectional (surveys)
Analytical, e.g. case-control; cohort
Experimental (also analytical)
Randomized Clinical Trials
Community Intervention Studies

10. 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

11. 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: case-studies; cross-sectional studies
Analytical: used to test hypotheses: small / large scale. Examples: case-control, cross-sectional, cohort.

12. 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

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

14. 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

15. 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

16. 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

17. 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)

18. 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)

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

20. References
Last J. A dictionary of epidemiology. 5th 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.