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Study Types & Terminology

Study Types

  • Systematic Reviews summarize the results of a systematic literature search on a specific clinical question to develop clinical recommendations. The studies are reviewed, assessed, and the results are summarized according to the predetermined criteria of the review question. They assess the methodology, sample size, and quality of the studies, using the highest quality data available to answer specific clinical questions and develop practice recommendations.
  • Meta-Analysis takes this process one-step further, reviewing a clinical question for which multiple systematic reviews exist and combining all the results using accepted statistical methodology  
  • Randomized, Controlled Clinical Trial - A prospective, analytical, experimental study using primary data generated in the clinical environment. Individuals who are similar at the beginning are randomly allocated to two or more groups (treatment and control) and the outcomes of the groups are compared after sufficient follow-up time.
  • A study that shows the efficacy of a diagnostic test is called a prospective, blind comparison to a gold standard study. This is a controlled trial that looks at patients with varying degrees of an illness and administers both diagnostic tests -- the test under investigation and the "gold standard" test -- to all of the patients in the study.
  • Cohort Studies identify a large population who already has a specific exposure or treatment, follows them over time (prospective), and compares outcomes with another group that has not been affected by the exposure or treatment being studied. Cohort studies are observational and not as reliable as randomized controlled studies, since the two groups may differ in ways other than in the variable under study.
  • Case Control Studies are studies in which patients who already have a specific condition or outcome are compared with people who do not. Researchers look back in time (retrospective) to identify possible exposures. They often rely on medical records and patient recall for data collection. These types of studies are often less reliable than randomized controlled trials and cohort studies because showing a statistical relationship does not mean than one factor necessarily caused the other.
  • Case Series and Case Reports consist of collections of reports on the treatment of individual patients or a report on a single patient. Because they are reports of cases and use no control groups with which to compare outcomes, they have no statistical validity.
  • Background Information / Expert Opinion use varied evidence to present information that ranges from expert opinion to providing summaries of well-known information with established evidence. They are good resources to begin understanding a topic, learning definitions, and clinical parameters. However, when answering an EBP question, look for information with statistically significant data from resources higher-up in the pyramid.

Adapted from: Duke University Medical Center Library: Evidence-Based Medicine Resources

Retrospective vs. Prospective Studies 

Retrospective Studies The outcome of interest has already occurred by the time of enrollment in the study. Data are collected either from records or by asking participants to recall exposures. There is no follow-up of participants. 

Prospective Studies The outcome (and sometimes the exposure or intervention) has not occurred when the study begins. Participants are followed up over a period of time to observe outcomes.

Most cohort studies are prospective studies (though there may be retrospective cohorts), whereas case–control studies are retrospective studies. An interventional study is a prospective study since the investigator determines the exposure for each study participant and follows them to observe outcomes (Ranganathan et al., 2018).

Per Nickson (2019), retrospective studies are designed to analyze pre-existing data and may be subject to numerous biases. Retrospective studies may be based on chart reviews (data collection from the medical records of patients). Types of retrospective studies include:

  • case series
  • retrospective cohort studies (current or historical cohorts)
  • case-control studies 



Hess, D. R. (2004). Retrospective studies and chart reviews. Respiratory Care49(10), 1171–1174.

Nickson, C. (2019, January 9). Retrospective studies and chart reviews. Life in the Fast Lane • LITFL.

Ranganathan, P., & Aggarwal, R. (2018). Study designs: Part 1 - An overview and classification. Perspectives in Clinical Research9(4), 184–186.

Study Terminology

Research studies often contain language that may be unfamiliar to nurses. Per Chrisman et al., (2014) some of the basic terminology commonly found in research studies are:

Abstract: A brief summary or overview of the research study.
Case Study: A collection of detailed information about a person or specific group.
Causality: The relationship between the cause and effect of a situation or event.
Consent: An agreement given by a competent person to participate in a research study.
Control Group: Randomly selected participants from the research group who won't receive the experimental treatment or variable. Those in the control group will be compared with those receiving the treatment or variable under investigation to establish the effectiveness or ineffectiveness of the variable/treatment.
Double-Blind Study: A research study conducted in which neither the investigators nor the participants know specific details of the study, such as whether a placebo or trial medication is being administered.
Experimental Research Group: Randomly selected participants from the research group who will receive the experimental treatment, medication, or variable.
Hypothesis: An educated guess about the expected outcome of a study.
Mean: The average score between two variables or outcomes.
Single-Blind Study: A research study conducted in which the investigators know specific details of the study but the participants don't.
Variable: An intervention, action, or medication that's being studied to observe its effect on the research group. For example, a group of oncology patients may receive a new cancer medication to study its effectiveness on reducing tumor cell growth. In this scenario, the new medication is the intervention or variable.


Chrisman, J., Jordan, R., Davis, C., & Williams, W. (2014). Exploring evidence-based practice research. Nursing Made Incredibly Easy, 12(4), 8–  12. 

More Resources

Study Designs - Centre for Evidence-Based Medicine. This short article from the Centre for Evidence-Based Medicine, University of Oxford, gives a brief guide to the different study types and a comparison of the advantages and disadvantages.