GLOSSARY OF TERMS

P

Paired design
A study in which participants or groups of participants are matched (e.g. based on prognostic factors) and one member of each pair is allocated to the experimental  (intervention) group and the other to the control group.

Parallel group trial (synonym: independent group design)
A trial that compares two groups of people, one of which receives the intervention of  interest and one of which is a control group. Some parallel trials have more than two comparison groups and some compare different interventions without including a  non-intervention control group.

Peer review
A refereeing process used to check the quality and importance of reports of research. An  article submitted for publication in a peer reviewed journal is reviewed by other experts  in the area. It aims to provide a wider check on the quality and interpretation of a report and to improve its quality. See also External peer reviewer.

Performance bias
Systematic differences in care provided apart from the intervention being evaluated. For example, if patients know they are in the control group they may be more likely to use other forms of care, patients who know they are in the experimental (intervention) group may experience placebo effects, and care providers may treat patients differently  according to what group they are in. Blinding of study participants (both the recipients  and providers of care) is used to protect against performance bias.

Phase I studies
The first stage in testing a new drug in humans. Usually performed on healthy volunteers without a comparison group.

Phase II studies
Second stage in testing a new drug in humans. Often performed on healthy volunteers. These  are sometimes randomized controlled trials.

Phase III studies
Studies that are a full-scale evaluation of treatment. After a drug has been shown to be reasonably effective, it is essential to compare it to the current standard treatments for the same condition. Phase III studies are often randomized controlled trials.

Phase IV studies
Studies that are concerned with post-marketing surveillance. They are often promotional  exercises aimed at bringing a new drug to the attention of a large number of clinicians, and may be of limited scientific value.

Placebo
An inactive substance or procedure administered to a patient, usually to compare its effects with those of a real drug or other intervention, but sometimes for the  psychological benefit to the patient through a belief that s/he is receiving treatment. Placebos are used in clinical trials to blind people to their treatment allocation.  Placebos should be indistinguishable from the active intervention to ensure adequate blinding.

Placebo effect
A favorable response to an intervention, regardless of whether it is the real thing or a  placebo, attributable to the expectation of an effect, i.e. the power of suggestion. The  effects of many healthcare interventions are attributable to a combination of both placebo  and "active" (non-placebo) effects.

Point estimate
The results (e.g. mean, weighted difference, odds ratio, relative risk or risk difference)  obtained in a sample (a study or a meta-analysis) which are  used as the best estimate of what is true for the relevant population from which the  sample is taken. A confidence interval is a measure of the uncertainty (due to the play of chance) associated with that estimate.

Positive study
A term used to refer to a study with results indicating a beneficial effect of the  intervention being studied. The term can generate confusion because it can refer to both statistical significance and the direction of effect, studies often have multiple outcomes, the criteria for classifying studies as negative or positive are not always clear and, in the case of studies of risk or undesirable effects, "positive" studies are ones that show a harmful effect. See also Negative Study

Precision

1. A measure of the likelihood of random errors in the results  of a study, meta-analysis or measurement. Confidence intervals around the estimate of effect from each study are a measure of precision, and the weight given to the results of each study  in a meta-analysis (typically the inverse of the variance of the estimate of effect) is a measure of precision (i.e. the degree to which a study  influences the overall estimate of effect in a meta-analysis is determined by the precision of its estimate of effect).
2. The proportion of relevant citations located using a specific search strategy, i.e. the  number of relevant studies (meeting the inclusion criteria for a trials register or a review) divided by the total number of  citations retrieved.

Prevalence
The number of existing cases of a particular disease or condition in a given population at  a designated time.

Prevalence study
See cross-sectional study.

Primary study (synonyms: included study, original study)
"Original research" in which data is first collected; a study included in a systematic review. The term primary research is sometimes used to distinguish it from "secondary research" (reanalysis of previously  collected data), meta-analysis, and other ways of combining  studies (such as economic analysis and decision analysis).

Probability distribution
The function that gives the probabilities that a variable equals each of a sequence of  possible values. Examples include the binomial, chi square, normal and Poisson distributions.

Prospective study
In evaluations of the effects of healthcare interventions, a study in which people are divided into groups that are exposed or not exposed to the intervention(s) of interest before the outcomes have occurred. Randomized controlled trials are always prospective studies and case control studies never are. Concurrent cohort studies  are prospective studies, whereas historical cohort studies are not (see cohort study), although in Epidemiology a prospective study is sometimes used as a synonym for cohort study. See also Retrospective Study.

Protocol
The plan or set of steps to be followed in a study.

Publication bias
A bias in the published literature where the publication of research depends 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.

P-value
The probability (ranging from zero to one) that the observed results in a study, or  results more extreme, could have occurred by chance. In a meta-analysis the P-value for  the overall effect assesses the overall statistical significance of the difference between the treatment and control groups, whilst the P-value for the heterogeneity statistic assesses the statistical significance of differences between the effects observed in each study.