Safety: Substantive evidence of an absence of harm.2

Sample: A subset of a population.4

Sampled population: The population from which the sample was taken.4

Selection bias: Systematic differences in groups that are compared (affects internal validity). Random allocation and adequate concealment of allocation protects against this bias.6, 11 Selection bias may also occur with systematic differences between those who are selected for study and those who are not (affects external validity). Selection bias may also apply to how studies are selected for inclusion in systematic reviews.

Sensitivity: 1) The proportion of time a diagnostic test is positive in individuals who have the disease or condition.1, 4, 6 A sensitive test has a low false-negative rate. Calculation: Sensitivity = TP / (TP + FN), where TP = true positive and FN = false negative. Also called true positive rate or detection rate. 2) In systematic review, sensitivity applies to article identification and is a measure of a search’s ability to correctly identify relevant articles.6 Calculation: sensitivity = number of relevant articles identified by the search / total number of relevant articles from all searches. Also called recall.

Sensitivity analysis: An analysis used to determine how sensitive the results of a study or systematic review are to changes in how it was done.6 Sensitivity analysis is used to assess how robust the results are to uncertain decisions or assumptions about the data and the methods that were used.

Serious adverse event: Any adverse event with serious medical consequences, including death, hospital admission, prolonged hospitalization, and persistent or significant disability or incapacity.2

Severe adverse event: An adverse event that is severe (including “non-serious” adverse events).2 For example, a rash could be “severe,” but not “serious” (i.e., not resulting death, hospital admission, prolonged hospitalization, and persistent or significant disability or incapacity).

Side effects: Unintended drug effects (beneficial or harmful) when given at doses normally used for therapeutic effects.2

Single blind: See blind study.

Specificity: The proportion of time a diagnostic test is negative in individuals who do not have the disease or condition.1,4,6 A specific test has a low false-positive rate. Calculation: Specificity = TN / (TN + FP), where TN = true negative and FP = false positive.

Standard of care: The typical or usual treatment for a particular condition at that time.6 Also called conventional or standard treatment.

Strength of evidence: An evaluation of a body of evidence. Core domains include risk of bias, consistency, directness, and precision.55 Strength of evidence grades for the Agency for Healthcare Research and Quality Effective Health Care Program’s comparative effectiveness reviews are high, moderate, low, or insufficient. The high, medium, and low grades indicate the level of confidence that the evidence reflects the true effect. An insufficient grade indicates that evidence is unavailable or inconclusive.

STROBE: Acronym for Strengthening The Reporting of Observational Studies in Epidemiology.56 The STROBE statement provides guidelines for reporting observational studies in epidemiology.

Summary guides: Plain-language guides for clinicians, consumers, or policymakers that summarize the findings of research reviews on the benefits and harms of different treatment options; one of the products from the Agency for Healthcare Research and Quality’s Effective Health Care Program.15 The John M. Eisenberg Center translates comprehensive evidence reports (research reviews) into short guides (called consumer guides, clinician guides, and policymaker summaries).

Surrogate endpoint: Measurements of a patient’s physical or biomedical status used as a surrogate for, or to infer the degree of, disease (e.g., blood pressure as a surrogate for stroke or heart attack).1, 6 Surrogate endpoints correlate with clinical outcomes but the relationship is not necessarily definitive. Also called clinical intermediary, intermediate outcome, or surrogate outcome.

Synthesis: See evidence synthesis.

Systematic error: Measurement error introduced into a study by its design, rather than due to random variation.4, 7 A systematic error is the same (or constant) over all observations. See also bias.

Systematic overview: See systematic review.

Systematic review: A structured literature review conducted in a systematic fashion using preset criteria and a protocol.1 More specifically, it is a scientific investigation that focuses on a specific question and uses explicit, prespecified scientific methods to identify, select, assess, and summarize the findings of similar but separate studies (qualitative synthesis).11 Systematic review is a type of evidence synthesis that may include quantitative synthesis (meta-analysis), depending on the available data.11 See also evidence synthesis and meta-analysis.