1. Types of study designs Arash Najimi
PhD. Candidate Department of health education & health promotion
Isfahan University of Medical Sciences

2. Types of Studies Descriptive Studies Observational Analytic Studies
Cross Sectional studies
Ecologic studies
Case Control studies
Cohort studies
Experimental Studies
Randomized controlled trials

3. Hierarchy of Study Types
Analytic
Descriptive
Case report
Case series
Survey
Observational
Cross sectional
Ecologic
Case-control
Cohort studies
Experimental
Randomized controlled trials
Field Trials
Community Trials
Strength of evidence for causality between a risk factor and outcome

4. Descriptive studies Getting a “lay of the land”
Surveys (NHIS, MCBS)
Describing a novel phenomena
Case reports or case series

5. Descriptive studies Cannot establish causal relationships
Still play an important role in describing trends and generating hypotheses about novel associations
The start of HIV/AIDS research
Squamous cell carcinoma in sexual partner of Kaposi sarcoma patient. Lancet Jan 30;1(8266):286.
New outbreak of oral tumors, malignancies and infectious diseases strikes young male homosexuals. CDA J Mar;10(3):39-42.
AIDS in the "gay" areas of San Francisco. Lancet Apr 23;1(8330):923-4.

6. Analytic Studies
Attempt to establish a causal link between a predictor/risk factor and an outcome.
You are doing an analytic study if you have any of the following words in your research question:
greater than, less than, causes, leads to, compared with, more likely than, associated with, related to, similar to, correlated with

7. Hierarchy of Study Types
Analytic
Descriptive
Case report
Case series
Survey
Observational
Cross sectional
Ecologic
Case-control
Cohort studies
Experimental
Randomized controlled trials
Field Trials
Community Trials
Strength of evidence for causality between a risk factor and outcome

8. Cross-sectional Study: Pluses
+ Prevalence (not incidence)
+ Fast/Inexpensive - no waiting!
+ No loss to follow up
+ Associations can be studied

9. Cross-sectional study: minuses
- Cannot determine causality
Cigarette smoking
Depression
time

10. Cross-sectional study: minuses
- Cannot determine causality
- Cannot study rare outcomes

11. Case control studies
Investigator works “backward” (from outcome to predictor)
Sample chosen on the basis of outcome (cases), plus comparison group (controls)

12. Study multiple exposures in a Case-control StudyB
Exposed
to A
Not Exposed
Disease
No Disease

13. Case control studies
Determines the strength of the association between each predictor variable and the presence or absence of disease
Cannot yield estimates of incidence or prevalence of disease in the population (why?)
Odds Ratio is statistics

14. Case-control Study: pluses
+ Rare outcome/Long latent period
+ Inexpensive and efficient: may be only feasible option
+ Establishes association (Odds ratio)
+ Useful for generating hypotheses (multiple risk factors can be explored)

15. Case-control study-minuses
Causality still difficult to establish
Selection bias (appropriate controls)
Recall bias: sampling (retrospective)
Cannot tell about incidence or prevalence

16. Cohort studies
A cohort (follow-up, longitudinal) study is a comparative, observational study in which subjects are grouped by their exposure status, i.e., whether or not the subject was exposed to a suspected risk factor
The subjects, exposed and unexposed to the risk factor, are followed forward in time to determine if one or more new outcomes (diseases) occur
Subjects should not have outcome variable on entry
The rates of disease incidence among the exposed and unexposed groups are determined and compared.

17. Study multiple outcomes in a cohort Study
Exposed
Not Exposed C B
Develop
Disease A
Do not

18. Elements of a cohort study
Selection of sample from population
Measures predictor variables in sample
Follow population for period of time
Measure outcome variable
Famous cohort studies
Framingham
Nurses’ Health Study
Physicians’ Health Study
Olmsted County, Minnesota

19. Prospective cohort study structure
The present
The future
Top USMLE scorers
Everyone else
time

20. Strengths of cohort studies
Know that predictor variable was present before outcome variable occurred (some evidence of causality)
Directly measure incidence of a disease outcome
Can study multiple outcomes of a single exposure (RR is measure of association)

21. Weaknesses of cohort studies
Expensive and inefficient for studying rare outcomes
Often need long follow-up period or a very large population
Loss to follow-up can affect validity of findings

22. Other types of cohort studies
Retrospective cohort
Identification of cohort, measurement of predictor variables, follow-up and measurement of outcomes have all occurred in the past
Much less costly than prospective cohorts
Investigator has minimal control over study design

23. Other types of cohort studies
Nested case-control study
Case-control study embedded in a cohort study
Controls are drawn randomly from study sample
Case cohort Study

24. Hierarchy of Study Types
Analytic
Descriptive
Case report
Case series
Survey
Observational
Cross sectional
Ecologic
Case-control
Cohort studies
Experimental
Randomized controlled trials
Field Trials
Community Trials
Strength of evidence for causality between a risk factor and outcome

25. Randomized controlled trials
Investigator controls the predictor variable (intervention or treatment)
Major advantage over observational studies is ability to demonstrate causality
Randomization controls unmeasured confounding
Only for mature research questions

26. Basic Trial Design Treatment Dx No Dx Control Dx No Dx Placebo
Population
Treatment
Dx No Dx
Randomization
Sample
Control
Dx No Dx
Placebo
For example, select persons with high LDL-cholesterol from among patients presenting to the metabolic clinic at the SFVA and treat with lovastatin to see if this results in lower LDL cholesterol.
Note can have an uncontrolled trial or time series design. These have no concurrent control group and can produce the wrong answer due to learning, regression to the mean or secular trends. Note, for example, that it is common to have a 30-50% improvement in depression scores, pain scores, hot flash scores.
Can have a controlled trial without randomization. This addresses learning, regression to the mean and secular trends, but introduces the issue of baseline differences in the groups. This design is really no different from a double cohort study.
Can have a randomized controlled trial without placebo (blinding). This allows (as will be discussed later) for co-intervention and biases outcome ascertainment.
Also note that these three elements - treatment, randomization and blinding result in the major ethical concerns regarding randomized trials.

27. Steps in a randomized controlled trial
Select participants
Measure baseline variables
Randomize
Eliminates baseline confounding
Types (simple, stratified, block)

28. Steps in a randomized controlled trial
Blinding the intervention
As important as randomization
Follow subjects
Measure outcome
Clinically important measures
Adverse events

29. Comparing Cohort Studies with Randomized Trials
Interventional Study
Observational Study
Study group
Study group
Random Allocation
No Allocation
Cannot randomize people to receive a putatively harmful substance, such as a suspected carcinogen, the “exposure” in most randomized trials is a treatment or preventive measure
Critical to interpreting study findings
In a nonrandomized study, when we observe an association of an exposure with a disease, we are left with uncertainty as to whether the association may be a result of the fact that people were NOT randomized to the exposure;
Perhaps it is not the exposure, but rather the factors that led people to be exposed, that are associated with the disease.
E.g., If an increased risk of a disease is found in workers at a certain factory, and if workers at this factory tend to live in a certain area, the increased risk of disease could result from an exposure associated with their place of residence rather than with their occupation or place of work.
Group A
Group B
Group A
Group B

30. Hierarchy of Study Types
Analytic
Descriptive
Case report
Case series
Survey
Observational
Cross sectional
Ecologic
Case-control
Cohort studies
Experimental
Randomized controlled trials
Field Trials
Community Trials
Strength of evidence for causality between a risk factor and outcome