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A term often used in clinical research is statistical power. The power of a statistical test is the ability of the test to detect an effect, if the effect actually exists. In statistical terms, it is the probability that it will correctly lead to the rejection of a null hypothesis.

In some cases we may not be able to reject the null hypothesis, not because it is true, but because we do not have sufficient evidence against it. This might be because the experiment is not large enough to reject the null hypothesis. As such, the power of a test can be described as the probability of not making a Type II error (not rejecting the null hypothesis when in fact it is false).


A substance or cellular component from which another substance or cellular component is formed.

Predictive genetic test

A predictive genetic test is a genetic test in a person without symptoms to predict future risk of disease. This testing allows early identification of individuals at risk of a specific condition, which can lead to reduced risk through targeted screening and prevention. For example, a woman who is found to have a BRCA1 or BRCA2 gene is at increased risk of breast cancer. She might be offered regular breast screening, or even preventative surgery, to help reduce her risk.

The value of a predictive test depends on the nature of the disease for which testing is being carried out, how effective treatment is, and the cost and efficacy of screening and surveillance measures.

Predictive Medicine

Predictive medicine is a field of medicine that predicts the probability of disease. When an individual is predicted to have a high risk of a disease, preventive measures can be started in order to either prevent the disease altogether or significantly decrease its impact upon the patient. Preventive measures might be lifestyle modifications and/or increased monitoring by healthcare professionals.

Predictive medicine changes medicine from being reactive to being proactive, and has the potential to extend healthy lifetimes and to prevent disease. As yet it is not possible to predict with 100% certainty that a specific disease will occur. Predictive genetic testing is one of the key approaches in predictive medicine.


Someone who is predisposed to a disease is more likely than other people to develop the disease in the future.

For example, someone who is genetically predisposed to develop Alzheimer's has a genetic makeup that increases their risk of developing this disease. A predisposition will not in itself cause the disease, but the disease may eventually be triggered by particular environmental or lifestyle factors, such as tobacco smoking or diet. Genetic testing is able to identify individuals who are genetically predisposed to certain diseases.


Prevalence is the proportion of a population found to have a condition (typically a disease or a risk factor such as smoking). It is calculated by comparing the number of people found to have the condition with the total number of people studied, and is usually expressed as a fraction (for example, 1/3), as a percentage (%) or as the number of cases per 10,000 or 100,000 people.

Prevalence can be measured at a particular point in time (point prevalence), or over a specified period such as a year (period prevalence).


Probability is the measure of the likelihood that a particular event will occur.

Probability is quantified as a number between 0 and 1 (where 0 indicates impossibility and 1 indicates certainty). The higher the probability of an event, the more certain we are that the event will occur. A simple example is the toss of a fair (unbiased) coin. Since the two outcomes are equally probable, the probability of 'heads' is equal to the probability of 'tails'. Therefore, the probability of either 'heads' or 'tails' is 1/2 (or 50%).

Proof of concept

A proof of concept (POC) trial is one type of trial carried out early in the clinical development phase of a medicine (in humans). Phase II trials usually begin with a proof of concept trial, which aims to show that the medicine interacts with its intended target and affects the disease in question.

Proof of mechanism

A proof of mechanism (POM) study is normally done within Phase I Clinical Development in healthy volunteers. Such studies are designed to show that a new medicine reaches its target organ(s), interacts with its molecular target, and affects the biology of the target cells as intended.

Prophylactic vaccine

Preventative (prophylactic) vaccines are substances that prepare the immune system to fight pathogens and provide immunity to a disease.

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