Types of study designs

Arash NajimiArash Najimi

PhD. CandidatePhD. CandidateDepartment of health education & health Department of health education & health

promotionpromotionIsfahan University of Medical SciencesIsfahan University of Medical Sciences

Types of Studies

Descriptive Studies

Observational Analytic Studies Cross Sectional studies Ecologic studies Case Control studies Cohort studies

Experimental Studies Randomized controlled trials

Hierarchy of Study Types

Descriptive•Case report•Case series•Survey

Analytic

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

Descriptive studies

Getting a “lay of the land” Surveys (NHIS, MCBS)

Describing a novel phenomena Case reports or case series

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. 1982 Jan 30;1(8266):286. New outbreak of oral tumors, malignancies and infectious

diseases strikes young male homosexuals. CDA J. 1982 Mar;10(3):39-42.

AIDS in the "gay" areas of San Francisco. Lancet. 1983 Apr 23;1(8330):923-4.

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

Hierarchy of Study Types

Descriptive•Case report•Case series•Survey

Analytic

Observational•Cross sectional•Ecologic•Case-control•Cohort studies

Strength of evidence for causality between a risk factor and outcome

Experimental•Randomized controlled trials•Field Trials•Community Trials

Cross-sectional Study: Pluses

+ Prevalence (not incidence)

+ Fast/Inexpensive - no waiting!

+ No loss to follow up

+ Associations can be studied

Cross-sectional study: minuses

time

- Cannot determine causality

Depression

Cigarette smoking

Cross-sectional study: minuses

- Cannot determine causality

- Cannot study rare outcomes

Case control studies

Investigator works “backward” (from outcome to predictor)

Sample chosen on the basis of outcome (cases), plus comparison group (controls)

Study multiple exposures in a Case-control Study

Disease No Disease

CB

Exposed to A

CB

Not Exposedto A

CB

Exposed to A

CB

Not Exposedto A

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

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)

Case-control study-minuses

- Causality still difficult to establish

- Selection bias (appropriate controls)

- Recall bias: sampling (retrospective)

- Cannot tell about incidence or prevalence

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.

Study multiple outcomes in a cohort Study

Exposed Not Exposed

CB

DevelopDisease A

CB

Do not Develop

Disease A

CB

DevelopDisease A

CB

Do not Develop

Disease A

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

time

The present The future

Top USMLE scorers

Everyone else

Prospective cohort study structure

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)

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

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

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

Hierarchy of Study Types

Descriptive•Case report•Case series•Survey

Analytic

Observational•Cross sectional•Ecologic•Case-control•Cohort studies

Strength of evidence for causality between a risk factor and outcome

Experimental•Randomized controlled trials•Field Trials•Community Trials

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

Basic Trial Design

PopulationPopulation

SampleSample

TreatmentTreatment Dx No Dx

ControlControl Dx No DxPlaceboPlacebo

RandomizationRandomization

Steps in a randomized controlled trial

1. Select participants2. Measure baseline variables3. Randomize

Eliminates baseline confounding Types (simple, stratified, block)

Steps in a randomized controlled trial

4. Blinding the intervention As important as randomization

5. Follow subjects6. Measure outcome

Clinically important measures Adverse events

Comparing Cohort Studies with Randomized Trials

Interventional Study Observational Study

Study group Study group

Random Allocation No Allocation

Group A Group B Group A Group B

Hierarchy of Study Types

Descriptive•Case report•Case series•Survey

Analytic

Observational•Cross sectional•Ecologic•Case-control•Cohort studies

Strength of evidence for causality between a risk factor and outcome

Experimental•Randomized controlled trials•Field Trials•Community Trials