P

Paired design
A trial 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
A trial that compares two groups of people concurrently, 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. (Also called independent group design.)

Participant
An individual who is studied in a trial, often but not necessarily a patient.

Performance bias
ystematic differences between intervention groups in care provided apart from the intervention being evaluated. For example, if participants know they are in the control group, they may be more likely to use other forms of care. If care providers are aware of the group a particular participant is in, they might act differently.  Blinding of study participants (both the recipients and providers of care) is used to protect against performance bias.

Phase I, II, III and IV trials
A series of levels of trials required of drugs before (and after) they are routinely used in clinical practice: Phase I trials assess toxic effects on humans (not many people participate in them, and usually without controls); Phase ll trials assess therapeutic benefit (usually involving a few hundred people, usually with controls, but not always); Phase III trials compare the new treatment against standard (or placebo) treatment (usually a full randomised controlled trial). At this point, a drug can be approved for community use. Phase IV monitors a new treatment in the community, often to evaluate long-term safety and effectiveness

Placebo
An inactive substance or procedure administered to a participant, usually to compare its effects with those of a real drug or other intervention, but sometimes for the psychological benefit to the participant 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.

Point estimate
The results (e.g. mean, weighted mean difference, odds ratio, risk ratio 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.

Population
The group of people being studied, usually by taking samples from that population. Populations may be defined by any characteristics e.g. geography, age group, certain diseases.

Positive association
See association

Positive study
A term used to refer to a trial 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.

Power
[In statistics:] The probability of rejecting the null hypothesis when a specific alternative hypothesis is true. The power of a hypothesis test is one minus the probability of Type II error.  In clinical trials, power is the probability that a trial will detect, as statistically significant, an intervention effect of a specified size. If a clinical trial had a power of 0.80 (or 80%), and assuming that the pre-specified treatment effect truly existed, then if the trial was repeated 100 times, it would find a statistically significant treatment effect in 80 of them.  Ideally we want a test to have high power, close to maximum of one (or 100%). For a given size of effect, studies with more participants have greater power. Studies with a given number of participants have more power to detect large effects than small effect.  (Also called statistical power.)

Pragmatic trial
A trial that aims to test a treatment policy in a 'real life' situation, when many people may not receive all of the treatment, and may use other treatments as well.  This is as opposed to an explanatory trial, which is done under ideal conditions and is trying to determine whether a therapy has the ability to make a difference at all (i.e. testing its efficacy)

Prevalence
The proportion of a population having a particular condition or characteristic: e.g. the percentage of people in a city with a particular disease, or who smoke.

Prevalence trial
A type of cross-sectional study that measures the prevalence of a characteristic.

Primary outcome
The outcome of greatest importance.

Primary study
‘Original research’ in which data are collected. The term primary study is sometimes used to distinguish it from a secondary study (re-analysis of previously collected data), meta-analysis, and other ways of combining studies (such as economic analysis and decision analysis). (Also called original study.)

Prospective study
In evaluations of the effects of healthcare interventions, a study in which people are identified according to current risk status or exposure, and followed forwards through time to observe outcome. Randomised controlled trials are always prospective studies. Cohort studies are commonly either prospective or retrospective, whereas case-control studies are usually retrospective. In Epidemiology, 'prospective study’ is sometimes misused as a synonym for cohort study. See also retrospective study.

Protocol
The plan or set of steps to be followed in a study. A Protocol for a systematic review should describe the rationale for the review, the objectives, and the methods that will be used to locate, select, and critically appraise studies, and to collect and analyse data from the included studies

Publication bias
See reporting bias

PubMed
A free access Internet version of MEDLINE also including records from before 1966 (old MEDLINE), some very recent records and some other life science journals.

P-value
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 the null hypothesis was true. In a meta-analysis, the P-value for the overall effect assesses the overall statistical significance of the difference between the intervention groups, whilst the P-value for the heterogeneity statistic assesses the statistical significance of differences between the effects observed in each study.