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Strongest Sources of Evidence

Systematic Review

A systematic review attempts to identify, appraise and synthesize all the empirical evidence that meets pre-specified eligibility criteria to answer a given research question. Researchers conducting systematic reviews use explicit methods aimed at minimizing bias, in order to produce more reliable findings that can be used to inform decision making (Cochrane Library Handbook Section 1.2.2). When such studies involve specifically quantitative techniques to combine and analyze data from multiple independent studies, they'd be referred to as a 'meta analysis.'


Works consisting of studies using a quantitative method of combining the results of independent studies (usually drawn from the published literature) and synthesizing summaries and conclusions which may be used to evaluate therapeutic effectiveness, plan new studies, etc. It is often an overview of clinical trials. It is usually called a meta-analysis by the author or sponsoring body and should be differentiated from reviews of literature (PubMed Medical Subject Heading).

All meta-analyses are systematic reviews, but not all systematic reviews are meta-analyses.

 Evidence-Based Guideline

Developed by any of a large number of different professional health care organizations, practices and agencies that systematically gather, appraise and combine health care evidence and create statements designed to assist practitioner and patient decision-making.

Other Types of Evidence

Here are descriptions used in CINAHL or MEDLINE (PubMed) databases to distinguish between different clinical study types and evidence-based information sources.  Other definitions can be found via the ‘Definitions’ tab of this research guide, and also in the many available texts covering the field of nursing and clinical research methods.

Randomized Controlled Trial

CINAHL Headings description: Experiments in which individuals are randomly allocated to an experimental or control group in order to test the value or efficiency of a treatment or intervention. For example, the experimental group is given a pharmacological agent being tested and the control group is given a drug in current use or a placebo and the results are compared.

 Controlled Clinical Trial

PubMed MeSH description: Work consisting of a clinical trial involving one or more test treatments, at least one control treatment, specified outcome measures for evaluating the studied intervention, and a bias-free method for assigning patients to the test treatment. The treatment may be drugs, devices, or procedures studied for diagnostic, therapeutic, or prophylactic effectiveness. Control measures include placebos, active medicine, no-treatment, dosage forms and regimens, historical comparisons, etc. When randomization using mathematical techniques, such as the use of a random numbers table, is employed to assign patients to test or control treatments, the trial is characterized as a RANDOMIZED CONTROLLED TRIAL.

See also: CINAHL Headings for defined types of controlled clinical trials: Double-Blind Studies; Intervention Trials; Preventive Trials; Randomized Controlled Trials; Single-Blind Studies; Therapeutic Trials; Triple-Blind Studies.

 Case Control Studies (Case-Comparison Studies)

CINAHL Headings description: Studies where individuals with a particular condition or disease (the cases) are selected for comparison with a series of individuals in whom the condition or disease is absent (the controls). Both groups are compared with respect to existing or past attributes or exposures thought to be relevant to the development of the condition or disease under study. The study proceeds from effect to cause.

 Cohort Studies (CINAHL: “Prospective Studies”)

PubMed MeSH description: Studies in which subsets of a defined population are identified. These groups may or may not be exposed to factors hypothesized to influence the probability of the occurrence of a particular disease or other outcome. Cohorts are defined populations which, as a whole, are followed in an attempt to determine distinguishing subgroup characteristics.

CINAHL Headings description: Studies where a population is selected on the presence and absence of certain characteristics (exposed and nonexposed) and followed over time to determine the incidence of disease or presumed effect. In CINAHL, the phrase 'Longitudinal Studies' maps to the CINAHL Heading "Prospective Studies," whereas in PubMed, 'Longitudinal Studies' is a MeSH term (appears beneath the broader MeSH term, "Cohort Studies," in the MeSH hierarchy).

 Meta Synthesis (Qualitative Synthesis)

CINAHL Headings description: An article that indicates the presence of a qualitative methodology that integrates results from a number of different, but inter-related studies.

Other Critically-Appraised Sources

CINAHL Evidence-Based Care Sheets

Click on the 'Evidence-Based Care Sheets' link located at the top of the CINAHL screen to find short overviews of evidence-based care recommendations covering 140 or more health care topics.