R

Random
Governed by chance. See randomisation.

Random allocation
A method that uses the play of chance to assign participants to comparison groups in a trial, e.g. by using a random numbers table or a computer-generated random sequence.  Random allocation implies that each individual or unit being entered into a trial has the same chance of receiving each of the possible interventions. It also implies that the probability that an individual will receive a particular intervention is independent of the probability that any other individual will receive the same intervention. See also quasi-random allocation, randomisation.

Random effects model
[In meta-analysis:] A statistical model in which both within-study sampling error (variance) and between-studies variation are included in the assessment of the uncertainty (confidence interval) of the results of a meta-analysis. See also fixed-effect model. When there is heterogeneity among the results of the included studies beyond chance, random-effects models will give wider confidence intervals than fixed-effect models.

Random error
Error due to the play of chance. Confidence intervals and P-values allow for the existence of random error, but not systematic errors (bias).

Random permuted blocks
A method of randomisation that ensures that, at any point in a trial, roughly equal numbers of participants have been allocated to all the comparison groups.Permuted blocks are often used in combination with stratified randomisation.  Also called block randomisation.

Random sample
A group of people selected for a study that is representative of the population of interest. This means that everyone in the population has an equal chance of being approached to participate in the survey, and the process is meant to ensure that a sample is as representative of the population as possible. It has less bias than a convenience sample: that is, a group that the researchers have more convenient access to.  Randomised trials are rarely carried out on random samples

Randomisation (spelled randomization in US English)
The process of randomly allocating participants into one of the arms of a controlled trial. There are two components to randomisation: the generation of a random sequence, and its implementation, ideally in a way so that those entering participants into a study are not aware of the sequence (concealment of allocation). (Also called randomisation.)

Randomisation blinding
See concealment of allocation.

Randomised controlled trial (RCT) (Synomym: randomised clinical trial)
An experiment in which two or more interventions, possibly including a control intervention or no intervention, are compared by being randomly allocated to participants. In most trials one intervention is assigned to each individual but sometimes assignment is to defined groups of individuals (for example, in a household) or interventions are assigned within individuals (for example, in different orders or to different parts of the body)..

Rate
The speed or frequency of occurrence of an event, usually expressed with respect to time. For instance, a mortality rate might be the number of deaths per year, per 100,000 people.

RCT
See randomised controlled trial.

Regression analysis
A statistical modelling technique used to estimate or predict the influence of one or more independent variables on a dependent variable, e.g. the effect of
age, sex, and educational level on the prevalence of a disease.  Logistic regression and meta-regression are types of regression analysis. 

Relative Risk (RR) 
See risk ratio

Relative risk reduction
The proportional reduction in risk in one treatment group compared to another.  It is one minus the risk ratio. If the risk ratio is 0.25, then the relative risk reduction is 1-0.25=0.75, or 75%.

Reliability
Refers to the degree to which results obtained by a measurement procedure can be replicated. Lack of reliability can arise from divergences between observers or measurement instruments, or instability in the attribute being measured.

Reporting bias
A bias caused by only a subset of all the relevant data being available. The publication of research can depend on the nature and direction of the study results.  Studies in which an intervention is not found to be effective are sometimes not published. Because of this, systematic reviews that fail to include unpublished studies may overestimate the true effect of an intervention. In addition, a published report might present a biased set of results (e.g. only outcomes or sub-groups where a statistically significant difference was found.  (Also called publication bias.)

Retrospective trial
A study in which the outcomes have occurred to the participants before the study commenced. Case-control studies are usually retrospective, cohort studies sometimes are, randomised controlled trials never are.  See also prospective study.

Review

1. A systematic review.
2. A review article in the medical literature which summarises a number of different studies and may draw conclusions about a particular intervention.Review articles are often not systematic.Review articles are also sometimes called overviews.
3. To referee a paper. See referee, referee process, external peer reviewer.
   

Risk
The proportion of participants experiencing the event of interest. Thus, if out of 100 participants the event (e.g. a stroke) is observed in 32, the risk is 0.32. The control group risk is the risk amongst the control group. The risk is sometimes referred to as the event rate, and the control group risk as the control event rate.  However, these latter terms confuse risk with rate. Statistical texts in particular are happy to discuss risk of beneficial effects as well as adverse events.

Risk difference (RD) 
The difference in size of risk between two groups. For example, if one group has a 15% risk of contracting a particular disease, and the other has a 10% risk of getting the disease, the risk difference is five percentage points. (Also called absolute risk difference, absolute risk reduction.)

Risk factor
An aspect of a person's condition, lifestyle or environment that increases the probability of occurrence of a disease. For example, cigarette smoking is a risk factor for lung cancer.

Risk ratio
The ratio of risk in two groups. In intervention studies, it is the ratio of the risk in the intervention group to the risk in the control group. A risk ratio of one indicates no difference between comparison groups. For undesirable outcomes, a risk ratio that is less than one indicates that the intervention was effective in reducing the risk of that outcome. (Also called relative risk, RR.)

RR
See risk ratio

Run-in period
A period before randomisation when participants are monitored but receive no treatment (or they sometimes all receive one of the study treatments, possibly in a blind fashion). The data from this stage of a trial are only occasionally of value but can serve a valuable role in screening out ineligible or non-compliant participants, in ensuring that participants are in a stable condition, and in providing baseline observations. A run-in period is sometimes called a washout period if treatments that participants were using before entering the trial are discontinued.