Evidence-based Medicine Toolkit

84 Evidence-based Medicine ToolkitReproducibility (repeatability, reliability): the results of a test or measureare identical or closely similar each time it is conducted.Retrospective study: study design in which cases where individuals whohad an outcome event in question are collected and analysed after theoutcomes have occurred.Risk: the probability that an event will occur for a particular patient or groupof patients. Risk can be expressed as a decimal fraction or percentage(0.25 = 25%).Risk ratio: see Relative risk.Selection bias: a bias in assignment or selection of patients for a study thatarises from study design rather than by chance. This can occur when thestudy and control groups are chosen so that they differ from each otherby one or more factors that may affect the outcome of the study.Sensitivity: the proportion of people with disease who have a positivetest.Sensitivity analysis: a process of testing how sensitive a result would be tochanges in factors such as baseline risk, susceptibility, the patients’ bestand worst outcomes, etc.SnNout: when a sign/test has a high sensitivity, a negative result rules outthe diagnosis.Specificity: the proportion of people free of a disease who have a negativetest.Spectrum bias: a bias caused by a study population whose disease profiledoes not reflect that of the intended population (for example, if they havemore severe forms of the disorder).SpPin: when a sign/test has a high specificity, a positive result rules in thediagnosis.Stratification: division into groups. Stratification may also refer to a processto control for differences in confounding variables, by making separateestimates for groups of individuals who have the same values for theconfounding variable.Strength of inference: the likelihood that an observed difference betweengroups within a study represents a real difference rather than merechance or the influence of confounding factors, based on both p valuesand confidence intervals. Strength of inference is weakened by variousforms of bias and by small sample sizes.Survival curve: a graph of the number of events occurring over time or thechance of being free of these events over time. The events must be discreteand the time at which they occur must be precisely known. In mostclinical situations, the chance of an outcome changes with time. In mostsurvival curves the earlier follow up periods usually include results frommore patients than the later periods and are therefore more precise.