S

Safety
[of an intervention:] Refers to serious adverse effects, such as those that threaten life, require or prolong hospitalization, result in permanent disability, or cause birth defects. Indirect adverse effects, such as traffic accidents, violence, and damaging consequences of mood change, can also be serious.

Search strategy

1. The methods used by a reviewer to identify trials. This includes handsearching relevant journals, searching electronic databases, contacting drug companies, other forms of personal contact and checking reference lists.
2. The combination of terms used to identify studies in an electronic database such as MEDLINE.

Secondary outcome
An outcome used to evaluate additional effects of the intervention deemed a priori as being less important than the primary outcomes.

Secondary study
A study of studies: a review of individual studies (each of which is called a primary study). A systematic review is a secondary study.

Selection bias

1. Systematic differences between comparison groups in prognosis or responsiveness to treatment. Random allocation with adequate concealment of allocation protects against selection bias. Other means of selecting who receives the intervention are more prone to bias because decisions may be related to prognosis or responsiveness to treatment.
2. A systematic error in reviews due to how studies are selected for inclusion.  Reporting bias is an example of this.
3. A systematic difference in characteristics between those who are selected for study and those who are not. This affects external validity but not internal validity.

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

Sequential trial
A randomised trial in which the data are analysed after each participant’s results become available, and the trial continues until a clear benefit is seen in favour of one of the comparison groups, or it is unlikely that any difference will emerge.  The main advantage of sequential trials is that they are usually shorter than fixed size trials when there is a large difference in the effectiveness of the interventions being compared.  Their use is restricted to conditions where the outcome of interest is known relatively quickly.  In a group sequential trial, a limited number of interim analyses of the data are carried out at pre-specified times during recruitment and follow up, say 3-6 times in all.

Side effect
Any unintended effect of an intervention. Side effects are most commonly associated with pharmaceutical products, in which case they are related to the pharmacological properties of the drug at doses normally used for therapeutic purposes in humans. See also adverse effect

Single blind
(Also called single masked.)  See blinding.

Single case report
See case study.

SMD
See Standardised mean difference

Standard deviation
A measure of the spread or dispersion of a set of observations, calculated as the average difference from the mean value in the sample.

Standard error
The standard deviation of the sampling distribution of a statistic. Measurements taken from a sample of the population will vary from sample to sample. The standard error is a measure of the variation in the sample statistic over all possible samples of the same size. The standard error decreases as the sample size increases. (Also called SE.)

Standard treatment
See conventional treatment.

Standardised mean difference
The difference between two estimated means divided by an estimate of the standard deviation. It is used to combine results from studies using different ways of measuring the same concept, e.g. mental health. By expressing the effects as a standardised value, the results can be combined since they have no units.  Standardised mean differences are sometimes referred to as a d index.  (Also called SMD.)

Statistical power
See power.

Statistical significant
A result that is unlikely to have happened by chance. The usual threshold for this judgement is that the results, or more extreme results, would occur by chance with a probability of less than 0.05 if the null hypothesis was true. Statistical tests produce a p-value used to assess this.

Stratification
The process by which groups are separated into mutually exclusive sub-groups of the population that share a characteristic: e.g. age group, sex, or socioeconomic status. It is possible to compare these different strata to try and see if the effects of a treatment differ between the sub-groups. See also sub-group analysis.

Stratified randomisation
A method used to ensure that equal numbers of participants with a characteristic thought to affect prognosis or response to the intervention will be allocated to each comparison group. For example, in a trial of women with breast cancer, it may be important to have similar numbers of pre-menopausal and post-menopausal women in each comparison group. Stratified randomisation could be used to allocate equal numbers of pre- and post-menopausal women to each treatment group. Stratified randomisation is performed by performing separate randomisation (often using random permuted blocks) for each strata. See also minimisation.

Sub-group analysis
An analysis in which the intervention effect is evaluated in a defined subset of the participants in a trial, or in complementary subsets, such as by sex or in age categories. Trial sizes are generally too small for sub-group analyses to have adequate statistical power. Comparison of sub-groups should be by test of interaction rather than by comparison of p-values. Sub-group analyses are also subject to the multiple comparisons problem. See also multiple comparisons.

Surrogate endpoints
Outcome measures that are not of direct practical importance but are believed to reflect outcomes that are important; for example, blood pressure is not directly important to patients but it is often used as an outcome in clinical trials because it is a risk factor for stroke and heart attacks. Surrogate endpoints are often physiological or biochemical markers that can be relatively quickly and easily measured, and that are taken as being predictive of important clinical outcomes.  They are often used when observation of clinical outcomes requires long follow-up.  (Also called intermediary outcomes, surrogate outcomes.)

Surrogate outcomes
See surrogate endpoints

Survey
See cross-sectional study.

Survival analysis
The analysis of data that measure the time to an event e.g. death, next episode of disease.  See also time to event.

Sub-group analysis
An analysis in which the intervention effect is evaluated in a defined subset of the participants in a trial, or in complementary subsets, such as by sex or in age categories. Trial sizes are generally too small for sub-group analyses to have adequate statistical power. Comparison of sub-groups should be by test of interaction rather than by comparison of p-values. Sub-group analyses are also subject to the multiple comparisons problem. See also multiple comparisons.

Surrogate endpoints
Outcome measures that are not of direct practical importance but are believed to reflect outcomes that are important; for example, blood pressure is not directly important to patients but it is often used as an outcome in clinical trials because it is a risk factor for stroke and heart attacks. Surrogate endpoints are often physiological or biochemical markers that can be relatively quickly and easily measured, and that are taken as being predictive of important clinical outcomes.  They are often used when observation of clinical outcomes requires long follow-up.  (Also called intermediary outcomes, surrogate outcomes.)

Systematic error
See bias.

Systematic review (synonym: systematic overview)
A review of a clearly formulated question that uses systematic and explicit methods to identify, select, and critically appraise relevant research, and to collect and analyse data from the studies that are included in the review. Statistical methods (meta-analysis) may or may not be used to analyse and summarise the results of the included studies. See also Cochrane Review.