Glossary of terms

Person years

Person years describes the accumulated amount of time that all the people in the study were being followed up. So, if five people were followed up for ten years each, this would be equivalent to 50 person-years of follow up. Sometimes the rate of an event in a study is given per person year rather than as a simple proportion of people affected, to take into account the fact that different people in the study may have been followed up for different lengths of time.

Person years describes the accumulated amount of time that all the people in the study were being followed up. So, if five people were followed up for ten years each, this would be equivalent to 50 person-years of follow up. Sometimes the rate of an event in a study is given per person year rather than as a simple proportion of people affected, to take into account the fact that different people in the study may have been followed up for different lengths of time.

Phase I trials

Phase I trials are the early phases of drug testing in humans. These are usually quite small studies which primarily test the drug’s safety and suitability for use in humans, rather than its effectiveness. They often involve between 20 and 100 healthy volunteers, although they sometimes involve people who have the condition that the drug is aimed at treating. To test the drug’s safe dosage range, very small doses are given initially and are gradually increased until the levels suitable for use in humans are found.

These studies also test how the drug behaves in the body, examining how it is absorbed, where it is distributed, how it leaves the body and how long it takes to do this.

Phase II trials

During this phase of testing, a drug’s effectiveness in treating the targeted disease in humans is examined for the first time and more is learnt about appropriate dosage levels.

This stage usually involves 200 to 400 volunteers who have the disease or condition that the drug is designed to treat. The drug’s effectiveness is examined and more safety testing and monitoring of the drug’s side effects are carried out.

Phase III trials

In this phase of human testing of treatments, the effectiveness and safety of the drug undergoes a rigorous examination in a large, carefully controlled trial to see how well it works and how safe it is. The drug is tested in a much larger sample of people with the disease or condition than before, with some trials including thousands of volunteers. Participants are followed up for longer than in previous phases, sometimes over several years.

These controlled tests usually compare the new drug’s effectiveness with either existing drugs or a placebo. These trials are designed to give the drug as unbiased a test as possible to ensure that the results accurately represent its benefits and risks. The large numbers of participants and the extended period of follow-up give a more reliable indication of whether the drug will work and allows rarer or longer-term side effects to be identified.

Positive predictive value

This is one of a set of measures used to show how accurate a diagnostic test is (see sensitivity, specificity and negative predictive value). The positive predictive value (PPV) of a test is how well the test identifies people who have a disease. The PPV is the proportion of people with a positive test result who truly have the disease. For example, if a test has a PPV of 99%, this means that 99% of the people who test positive will have the disease, while 1% of those who test positive will not have the disease (false positives).

The PPV of a test varies depending on how common the disease is in the population being tested. A test’s PPV tends to be higher in populations where the disease is more common and lower in populations where the disease is less common.

Pre-clinical evaluations

These are in vitro (for example, in cell cultures) and in vivo laboratory animal tests on drugs in development, which are carried out to ensure that they are safe and effective before they go on to be tested in humans (clinical studies).

Prevalence

Prevalence describes how common a particular characteristic (for example, a disease) is in a specific group of people or population at a particular time. Prevalence is usually assessed using a cross sectional study.

Prospective observational study

This study identifies a group of people and follows them over a period of time to see how their exposures affect their outcomes. A prospective observational study is normally used to look at the effect of suspected risk factors that cannot be controlled experimentally, such as the effect of smoking on lung cancer.

Prospective study

A prospective study asks a specific study question (usually about how a particular exposure affects an outcome), recruits appropriate participants and looks at the exposures and outcomes of interest in these people over the following months or years.

Publication bias

Publication bias arises because researchers and editors tend to handle positive experimental results differently from negative or inconclusive results. It is especially important to detect publication bias in studies that pool the results of several trials.

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