Types of study designs

Types of study designs Arash Najimi PhD. CandidateDepartment of health education & health promotion Isfahan 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 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

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

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

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

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)

C C C C B B B B Not Exposed to A Exposed to A Not Exposed to A Exposed to A No Disease Disease Study multiple exposures in a Case-control Study

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.

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

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

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

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

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

Dx No Dx Control Basic Trial Design Treatment Population Dx No Dx Randomization Sample Placebo

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

Steps in a randomized controlled trial • Blinding the intervention • As important as randomization • Follow subjects • 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

Types of study designs