GLOSSARY OF TERMS

Q

Quality
See methodological quality.

Quasi-random allocation
A method of allocating participants to different forms of care that is not truly random;  for example, allocation by date of birth, day of the week, medical record number, month of  the year, or the order in which participants are included in the study (e.g. alternation).

Quasi-randomized trial
A trial using a quasi-random method of allocating participants to different forms of care. There is a greater risk of selection bias in quasi-random trials where allocation is not adequately concealed compared with randomized controlled trials with adequate concealment of allocation.

R

Random
Governed by chance. See randomization.

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 concealment of allocation, quasi-random allocation, randomization.

Random effects model
A statistical model sometimes used in meta-analysis 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 fixed effect model. If there is significant heterogeneity among the results of the included studies, random effects models will give wider confidence intervals than fixed effect models.

Random error (synonym: sampling error)
Error due to the play of chance. Confidence intervals and P-values represent the  probability of random errors, but not systematic errors (bias).

Random permuted blocks
A method of randomization 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 randomization.

Random selection (synonym: random sampling)
A method of obtaining a representative, unbiased group of people from a larger population.  Random selection which is not related to how participants are allocated to comparison groups is frequently used in cross-sectional and cohort studies, which are not randomized controlled trials, and it is frequently not used in randomized controlled trials. In older trial reports, however, the term is occasionally used instead of random allocation or randomization.

Randomization (spelled  randomization in US English)
Method used to generate a random allocation sequence, such as using tables of random numbers or computer-generated random sequences. The method of randomization should be distinguished from concealment of allocation because of the risk of selection bias despite  the use of randomization, if there is not adequate allocation concealment. For instance, a list of random numbers may be used to randomize participants, but if the list is open to  the individuals responsible for recruiting and allocating participants, those individuals  can influence the allocation process, either knowingly or unknowingly.

Randomization blinding
See concealment of allocation.

Randomized controlled trial (RCT) (Synonym: randomized clinical trial)
An experiment in which investigators randomly allocate eligible people into (e.g.  treatment and control) groups to receive or not to receive one or more interventions that are being compared. The results are assessed by comparing outcomes in the treatment and  control groups. NOTE: when using randomized controlled trial as a search term (publication type) in MEDLINE, the US spelling (randomized) must be used.

RCT
See randomized controlled trial.

Referee
See referee process.

Referee process
System by which a review goes out to editors and also sometimes one or more external parties with content, methodological or user expertise. These people are sometimes called external peer reviewers or referees.

Register of trials
See trials register.

Relative Risk (RR) (synonym: risk  ratio)
The ratio of risk in the intervention group to the risk in the control group. The risk  (proportion, probability or rate) is the ratio of people with an event in a group to the  total in the group. A relative risk of one indicates no difference between comparison groups. A relative risk of one indicates no difference between comparison groups. For undesirable outcomes a RR that is less than one indicates that the intervention was effective in reducing the risk of that outcome.

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.

Retrospective study
A study in which the outcomes have occurred to the participants before the study commenced. Case control studies are always retrospective, cohort studies sometimes are, randomized controlled trials never are.
See prospective study.

Review

1. A systematic review.
2. A review article in the medical literature, which summarizes 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.

RevMan (Review Manager)
Software developed for the Cochrane Collaboration to assist Reviewers in preparing Cochrane Reviews.  Reviewers enter their protocols and reviews  into RevMan.  

Risk difference (RD) (synonym:  absolute risk reduction)
The absolute difference in the event rate between two comparison groups. A risk difference  of zero indicates no difference between comparison groups. For undesirable outcomes a RD that is less than zero indicates that the intervention was effective in reducing the risk of that outcome.

Risk factor
Aspects 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.