Download to read offline
study designs.ppt
- 1. TYPES OF MEDICALSTUDIES Observational Experimental Clinical Trial Descriptive Pure experiment Community Trial Analytical Cross-Sectional Longitudinal Prospective Retrospective Ecological
- 2. • Investigator exercisescontrol over allocation of exposure • More powerful than observational studies for testing etiological hypotheses • For ethical reasons the possibilities of conducting experiments in humans is limited Experimental Studies
- 3. SAMPLE nonparticipants RANDOMIZATION TO GROUPS(2….n) INTERVENTION CONTROL GROUP GROUP lost to follow-up MEASURE MEASURE OUTCOME OUTCOME DESIGN OF A CLINICAL/COMMUNITY TRIAL
- 4. Clinical trial - Goldstandard in medicine – provides the greatest justification for concluding causality and is subject to the least number of problems or biases - Involves people - Controlled trials are studies in which the experimental dug or procedure is compared with another drug or procedure, sometimes a placebo and sometimes the previously accepted treatment - Uncontrolled trials – no comparison group
- 5. Controls - Independent concurrentcontrols - Double-blind trials - Trials with self-controls - Trials with external (historical) controls
- 6. Did investigator assign exposures? ObservationalStudy Experimental Study comparison group? randomize allocation? Randomized Controlled Trial Non- Randomized Controlled Trial Analytical Study Descriptive Study direction? Cross- sectional Study Case- Control Study Cohort Study no yes yes no yes no
- 7. Observational Studies • Investigatorobserves occurrence of condition/s in “self”-assigned groups of people • Often most practical and feasible • Carried out in more natural settings • Less control over study situation results more susceptible to distorting influence
- 8. Descriptive Survey • Setsout to describe a situation ex. Distribution of depression in a population in relation to sex, age and other characteristics Analytical Survey • Sets out to test hypotheses or detect associations ex. Identify factors that explain higher rates of depression among women
- 9. OBJECTIVES OF DESCRIPTIVE EPIDEMIOLOGiCSTUDIES • To permit evaluation of trends in health and disease and comparisons among countries and subgroups within countries; this objective includes monitoring of known diseases as well as the identification of emerging problems • To provide a basis for planning, provision, and evaluation of health services • To identify problems to be studied by analytic methods and suggest areas that may be fruitful for investigation (Friis & Seelers, 1999)
- 10. TYPES OF EPIDEMIOLOGICSTUDIES Observational Analytical Cross-Sectional Longitudinal Ecological Prospective Retrospective Cohort Case-Control
- 11. Directionality of studydesign retrospective outcome exposure prospective
- 12. Prospective Study • Thefirst step in a prospective study design is to identify the relevant group/s of people and collect information about their exposure history. • We then follow these people over time and measure the incidence of the outcome/s of interest.
- 13. Time Direction of Inquiry DISEASE EXPOSED DISEASEDISEASE POPULATION FREE PEOPLE DISEASE EXPOSED DISEASE Design of a COHORT Study
- 14. Cohort Study D1 D2 E1n1 E2 n2
- 15. Cohort study ofBritish Doctors Current smoker Non smoker (1951) (1951) Yes a b Death of lung cancer 4180 224 No c d 45820 39776 # at beginning of study 50000 40000 90000
- 16. Coronary heart diseaserisk and blood cholesterol
- 17. The main limitationof the prospective design is the time and cost involved especially when studying chronic diseases that may only become apparent years after the exposure/s of interest or may require years of exposure to “cause” the outcome.
- 18. Example: To investigate therisk of bowel cancer (annual incidence = 100-300/100,000 persons) would require a cohort of thousands of individuals to be followed for 10-15 years in order to identify a sufficient number of outcomes for a reliable estimation of the association.
- 19. Historical Prospective Design •An alternative strategy for the concurrent prospective study design in order to reduce time and costs. • This design requires identifying a defined cohort from some time in the past. The follow- up period is the time that elapsed since exposure status was determined until the present. • Incidence and risk measures can be estimated in the same manner as in a concurrent prospective study.
- 20. Historical Prospective Design Particularlyuseful when the exposure under investigation is “unique” in some way. e.g., occurred only in the past, occurred in specific group of people. Therefore this design is often applied to the study of acute environmental exposures. Example: thyroid cancer risk among people exposed to the Chernobyl nuclear-reactor accident
- 21. Historical Prospective Design Possiblesources of information about exposure status: • industrial worker records • military records • insurance companies or health care provider companies • registries of persons receiving specific medical treatment
- 22. Historical Prospective Design •Often difficult to obtain information about other exposures in the cohort. • Sometime difficult to obtain comprehensive list of those who experienced the outcome (less problematic in a mortality study)
- 23. Historical and concurrentcohort studies PAST PRESENT FUTURE Historical cohort Cases Follow-up assembled Concurrent cohort Cases Follow-up assembled
- 24. Historical Cohort Study Non-concurrentCohort Study Retrospective Cohort Study Retrolective Study Example: X-rays among TB patients and lung cancer 2000 1985 1945 1925 TB treatment Follow-up period P r e s e n t
- 25. Time Direction of Inquiry Startwith: EXPOSED CASES (people with EXPOSED disease) POPULATION EXPOSED CONTROLS (people without EXPOSED disease) Design of a CASE-CONTROL Study
- 26.
- 27. comparative strengths ofcohort and case-control studies CASE-CONTROL • Well suited to study rare outcomes • Can easily study multiple exposures • Efficient if long delay between exposure and outcome COHORT • Well suited to study rare exposures • Can easily study multiple outcomes • Provides direct measure of risk of outcome among exposed and unexposed persons • Begins with healthy persons thereby preventing “selective survival” bias
- 28. comparative limitations ofcohort and case-control studies CASE-CONTROL • Inefficient for rare exposures • Not well suited to study multiple outcomes • Unable to provide data on absolute risk • Time sequence of exposure and outcome can be unclear COHORT • Inefficient for rare outcomes • Not well suited to study multiple exposures • Difficult if long delay between exposure and outcome* • Lost-to-follow-up problem • Can be costly*
- 29. comparative limitations ofcohort and case-control studies CASE-CONTROL • Relies on information about past exposures that may be prone to bias COHORT • Assessment of exposure status may influence participant’s behavior • Changing diagnostic criteria may introduce bias
- 30. Design of aCROSS-SECTIONAL Study Begin with: DEFINED POPULATION gather data on exposure and disease Exposed Exposed Exposed Exposed Disease Disease Disease Disease
- 31. Cross Sectional Study •Involves a random sample of a dynamic population • Efficient for describing characteristics of the population • Efficient for generating new hypotheses • Provides prevalence estimates of the outcome • Not useful design for determining causal effects • Not efficient for studying rare outcomes or those of short-duration • Repeat cross-sectional surveys are useful for assessing the impact of a service • Relatively inexpensive and quick
- 32. • Cannot providedirect estimates of risk • Difficult to interpret temporality between exposure and outcome Cross Sectional Study
- 33.
- 34.
- 35.
- 36.