Absolute effects are differences between outcomes in the groups being compared. For example, if 10% (10 per 100) experience an outcome in one of the treatment comparison groups and 5% (5 per 100) experience that outcome in the other group, the absolute effect is 10% - 5% = a 5% difference.

Allocation is the assignment of participants in comparisons of treatments to the different treatments (groups) being compared.

Association or correlation is a relationship between two attributes, such as using a treatment and experiencing an outcome.

The average difference is used to express treatment differences for continuous outcomes, such as weight, blood pressure or pain measured on a scale. It is the difference between the average value for an outcome measure (for example kilograms) in one group and that in a comparison group.

Baseline risk is an estimate of the likelihood that an individual or group will experience a health problem before a treatment is used.

The certainty of the evidence is an assessment of how good an indication a systematic review provides of the likely effect of a treatment; i.e. the likelihood that the effect will be substantially different from what the studies found (different enough that it might affect a decision). Judgements about the certainty of the evidence are based on factors that reduce the certainty (risk of bias, inconsistency, indirectness, imprecision and publication bias) and factors that increase the certainty.

In the context of comparisons of treatments, chance is the occurrence of differences between comparison groups that are not due to treatment effects or bias. The play of chance (random error) can lead to incorrect conclusions about treatment effects if too few outcomes occur in studies.

A confidence interval is a statistical measure of a range within which there is a high probability (usually 95%) that the actual value lies. Wide intervals indicate lower confidence; narrow intervals greater confidence.

Contamination is the inadvertent application of a treatment allocated to one comparison group to people in another comparison group in treatment comparisons.

An explanatory study (sometimes called an ‘efficacy’ study) is designed to assess the effects of a treatment given in ideal circumstances, in contrast to a pragmatic study.

Fair comparisons of treatments are comparisons designed to minimize the risk of systematic errors (biases) and random errors (resulting from the play of chance).

An outcome is a potential benefit or harm of a treatment measured in a treatment comparison. An outcome measure is how the outcome is measured in a study.

A p-value is the probability (ranging from zero to one) that the results observed in a study (or results more extreme) could have occurred by chance if in reality there were no treatment differences.

A measurable, observable, or felt improvement in health or behaviour not attributable to the treatment administered.

A placebo is a treatment that does not contain active ingredients, which has been designed to be indistinguishable from the active treatment being assessed.

A pragmatic study (sometimes called an ‘effectiveness’ study) is designed to assess the effects of a treatment given in the circumstances of everyday practice.

Probability is the chance or risk of something, such as an outcome, occurring. See Risk

Relative effects are ratios. For example, if the probability of an outcome in the treatment group is 10% (10 per 100) and the probability of that outcome in a comparison group is 5% (5 per 100), the relative effect is 5/10 = 0.50.

The reliability of a claim or evidence about a treatment effect is the extent to which it is dependable or can be trusted. It should be noted that reliability often has a different meaning in the context of research, which is the degree to which results obtained by a measurement procedure can be replicated.

Risk is the probability of an outcome occurring. See Probability

A scale is an instrument for measuring or rating an outcome with a potentially infinite number of possible values within a given range, such as weight, blood pressure, pain or depression.

Statistical significance is a difference that is unlikely (below a specified level of confidence – typically 5%) to be explained by the play of chance.

A study is an investigation that uses specified methods to evaluate something. Different types of studies can be used to evaluate the effects of treatments. Some are more reliable than others.

A subgroup is a subdivision of a group of people; a distinct group within a group. For example, in studies or systematic reviews of treatment effects, questions are often asked about whether there are different effects for different subgroups of people in the studies, such as women and men, or people of different ages.

Surrogate outcomes are 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 studies because it is a risk factor for stroke and heart attacks.

A systematic review is a summary of research evidence (studies) that uses systematic and explicit methods to summarise the research. It addresses a clearly formulated question using a structured approach to identify, select, and critically appraise relevant studies, and to collect and analyse data from the studies that are included in the review.

A theory is a supposition or a system of ideas intended to explain something.

Treatment comparisons are studies of the effects of treatments.

A treatment is any intervention (action) intended to improve health, including preventive, therapeutic and rehabilitative interventions and public health or health system interventions.