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Physician Assistant Systematic Review

Defnitions of Types of Studies

Systematic Review -A comprehensive literature review that is conducted in a methodical manner. The data is collected according to a pre-prescribed protocol and synthesized through a meta-analysis. 

Randomized Controlled Trial - A study where people are randomly assigned to receive one of several interventions. Many times one of these interventions is the control where no intervention or a placebo intervention is administered. 

Cohort Study - A study that follows a group or groups (cohorts) that have particular risk factors over a period of time to evaluate their likelihood of contracting a disease or other outcome. 

Case-Control Study - A study that compares a group of patients with a disease or outcome to a control group. The study looks back to retrospectively compare the exposure to a risk factor(s) and the likelihood of contracting a disease or outcome. 

Cross-Sectional Survey - A study that collects data about a population of interest at one point in time. They are considered to be snapshots of the populations about which the information is gathered. 

Case Reports - A study that looks at an individual case. Case reports often describe:

  • Unique cases that cannot be explained by known diseases or syndromes
  • Cases that show an important variation of a disease or condition
  • Cases that show unexpected events that may yield new or useful information

For More Information: Introduction to Study Design

Spotting the Study Type

Following these three questions will help you determine what type of study you are looking at. 

  1. What is the aim of the Study? 
    • To describe the population  = Descriptive (Systematic Reviews and Meta-Analysis)
    • To quantify the relationship between factors (PICO Questions) = Analytic (Case reports, cohort studies, cross-sectional studies, Randomized Control Trials)
  2. If Analytic, was the invention randomly allocated? 
    • Yes = Randomized Controlled Trial
    • No = Observational (Case reports, cohort studies,  cross-sectional  studies, Randomized Control Trials)
  3. When were the outcomes determined?
    • Sometime after the exposure or intervention = Cohort Study
    • At the same time at the exposure or intervention = cross-sectional study or survey
    • Before the exposure was determined= case reports

For More information: Centre for Evidence-Based Medicine Study Design

Type of Health Studies in Order of Strength of Conclusion

Much of health research can be broken down into observational and experimental studies. In observational studies, scientists gather information on the current state of patients, but they do not intervene in the person's life. Where in experimental research, scientists do intervene or at least use statistical methods to mimic intervention. The strength of the conclusion favors more experimental study designs. 


For more information, read this article on

Level of Evidence

NHMRC Evidence Hierarchy: Designations of "Levels of Evidence" According to Type of Research Question

A systematic review will only be assigned a level of evidence as high as the studies it contains, excepting where those studies are of level II evidence. Systematic reviews of level II evidence provide more data than the individual studies and any meta-analyses will increase the precision of the overall results, reducing the likelihood that the results are affected by chance. Systematic reviews of lower level evidence present results of likely poor internal validity and thus are rated on the likelihood that the results have been affected by bias, rather than whether the systematic review itself is of good quality. Systematic review quality should be assessed separately. A systematic review should consist of at least two studies. In systematic reviews that include different study designs, the overall level of evidence should relate to each individual outcome/result, as different studies (and study designs) might contribute to each different outcome. 


Level Intervention Diagnostic Accuracy Prognosis Aetiology Screening Intervention
I A systematic review of level II studies A systematic review of level II studies A systematic review of level II studies A systematic review of level II studies A systematic review of level II studies
II A Randomized controlled trial A study of test accuracy with an independent, blinded comparison with a valid reference standard, among consecutive persons with a defined clinical presentation A prospective cohort study A prospective cohort study A randomized controlled trail

A pseudo-randomized controlled trial (i.e. alternate allocation o some other method)

A study of test accuracy with an independent, blinded comparison with a valid reference standard, among non-consecutive persons with a defined clinical presentation.  All or none All or none A pseudo-randomized controlled trial (i.e. alternate allocation or some other method)

A Comparative study with concurrent controls. 

  • Non-randomized experiential trial.
  • cohort study
  • Case-Control Study
  • Interrupted time series with a control group. 
A comparison with a reference standard that does not meet the criteria required for Level II and III-1 evidence.  Analysis of prognostic factors amongst persons in a single aim of a randomized controlled trial A Retrospective Cohort study

A comparative study with concurrent controls. 

  • Nonrandomized, experimental trial. 
  • Cohort study
  • Case-control study

A comparative study without concurrent controls:

  • Historical control Study
  • Two or more single arm study
  • Interrupted time series without a parallel control group
Diagnostic Case-control Study A Retrospective Cohort Study A case-control study

A comparative study without concurrent controls:

  • Historical control study
  • two or more single arm study 
IV Case series with either post-test or pre-test/post-test outcomes Study of diagnostic yield (no reference standard) Case series, or cohort study of persons at different stages of disease A cross-sectional study or case series case series

Information taken from: NHMRC additional levels of evidence and grades for recommendations for developers of guidelines.