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Absolute effects

Absolute effects are differences between outcomes in the groups being compared. For example, if 10% (10 per 100) experience an outcome in one of the treatment comparison groups and 5% (5 per 100) experience that outcome in the other group, the absolute effect is 10% - 5% = a 5%  difference.


An artery is a tube that caries blood away from the heart.


An association is a link or connection.

Average difference

The average difference is used to express treatment effects for outcomes such as weight or pain that are measured using a scale. It is the difference between the average value for an outcome measure in one group and that in the comparison group.

Baseline risk

Baseline risk is an estimate of the likelihood that an individual or group will experience a health problem before a treatment is used.


The basis of a claim is what supports it.


A systematic error that may affect the results of a study because of weaknesses in its design, analysis or reporting.


In treatment comparisons, to blind someone means to keep them from knowing who received which treatment.

Blood pressure

A measure of how hard your heart is pumping to move blood through your body. People with high blood pressure are more likely to have a stroke or a heart attack.


A claim is something someone says that could be right, but could be wrong.

Comparison of treatments

A comparison of treatments is a study where researchers look carefully at the differences in what happens between groups that use different treatments.

Confidence interval

A confidence interval is a range that reflects the extent to which the play of chance may be responsible for a result from a study, such as an effect estimate. There is a high probability (usually 95%) that the actual effect estimate is within that range.


In treatment comparisons, any factors other than the treatments being compared which may affect the health outcomes being measured. For a factor to lead to confounding, it must differ between the treatment comparison groups, and affect the outcome of interest. An example could be age for treatments that aim to improve physical mobility. Older people will tend to have lower mobility irrespective of the treatment.


Contamination is the inadvertent application of a treatment allocated to one comparison group to people in another comparison group in treatment comparisons.


Feeling sad and depressed for weeks or months on end


When you are sick, doctors figure out what’s making you sick by asking questions and ordering tests. What they figure out is called a diagnosis.

Effect estimate

The most likely size of a treatment effect, based on the results of a study or a systematic review.


An effect of a treatment is something that it makes happen.


A treatment is effective if it causes something to happen that is wanted.

Eligibility criteria

Characteristics used to decide whether people are eligible to participate in a study and should be invited to participate


An event is when something happens, like someone getting sick or better.


Evidence is facts used to support what you believe or decide.

Explanatory study

An explanatory study (sometimes called an ‘efficacy study’) is designed to assess the effects of a treatment given in ideal circumstances, in contrast to a ‘pragmatic study’.

Fair comparison of treatments

A fair comparison of treatments is one where the only important difference between the groups that are compared is the treatments they receive.


In a comparison of treatments, fair means not giving an advantage to one treatment over another.

Faulty logic

Faulty logic is bad reasoning.

Good choice

A decision that uses the best information available at the time

Heart attack

When poor blood flow to a part of the heart results in damage to the heart muscle.


The kidneys are a pair of body parts that filter waste materials out of the blood.


The likelihood (or probability) of an event (outcome) is the chance that someone will experience that event; i.e. the number of people in a group that experience the event during a specified period, divided by the total number of people in that group.


In treatment comparisons, an outcome (or ‘outcome measure’) is something good or bad that can happen after a treatment and is measured in studies.


Diagnosis of a “disease” (e.g. through screening) which will never cause problems during a patient's lifetime


Unnecessary treatments (e.g. for a condition that causes no symptoms, and will go away on its own), or intensive treatments for a health condition that could be remedied with less intensive treatment.


For effect estimates, the p-value is the probability (ranging from 0 to 1) that the results observed in a study could have occurred by the play of chance, if the treatment actually had no impact on the outcome.

Placebo effect

Just believing that a treatment works can change how someone feels. This is called a placebo effect. Placebo effects are presumed to act psychologically through suggestion.


A placebo is a dummy or sham treatment that does not contain active ingredients, which has been designed to be indistinguishable from the active treatment(s) being assessed. It is used to blind participants and others involved in a study of treatment effects, and to reduce the risk of placebo effects.

Pragmatic study

A pragmatic study (sometimes called an ‘effectiveness’ study) is designed to assess the effects of a treatment given in the circumstances of everyday practice.


The extent to which errors due to the play of chance on the results of a study are likely to have occurred.


The document providing detailed plans for a study.


Also called "talk therapy" - treatment of psychological problems through communication and the relationship between an individual and a trained mental health professional

Random allocation

The process of assigning participants in a study to treatment comparison groups using a chance process, like drawing lots, to protect against bias.

Relative effects

Relative effects are ratios. For example, if the probability of an outcome in the treatment group is 10% (10 per 100) and the probability of that outcome in a comparison group is 5% (5 per 100), the relative effect is 5%/10%.


A scale is an instrument for measuring or rating an outcome with a potentially infinite number of possible values within a given range, such as weight, blood pressure, pain or depression.


Using tests to find out whether or not a person has a specific health condition before it causes any symptoms

Side effect

A side effect is a harmful or unpleasant effect of a treatment that is not planned.

Statistically significant

A “statistically significant” result is unlikely to have happened by chance. The usual threshold for this judgement is a probability of less than 5% (0.05).


When poor blood flow the brain results in damage to a part of the brain


A subgroup is a subdivision of a group of people; a distinct group within a group. For example, in studies or systematic reviews of treatment effects, questions are often asked about whether there are different effects for different subgroups of people in the studies, such as women and men, or people of different ages.

Surrogate outcome

Surrogate outcomes are outcome measures that are not of direct practical importance but are believed to reflect outcomes that are important. For example, blood pressure is not directly important to patients but it is often used as an outcome in studies because it is a risk factor for stroke and heart attacks.

Systematic review

A summary of studies addressing a clear question, using systematic and explicit methods to identify, select, and critically appraise relevant studies, and to collect and analyse data from them.

Treatment comparison group

In a study of the effects of treatments, a treatment comparison group is the group of people who receive one of the treatments being compared.

Treatment comparison

A treatment comparison is a study where researchers look carefully at the differences in what happens between groups that use different treatments.

Treatment effect

A treatment effect is something a treatment makes happen.


A treatment is any action intended to improve health.

Unfair comparison of treatments

An unfair comparison of treatments is one where there are important differences between the groups that are compared besides the treatments they receive.


In a comparison of treatments, unfair means giving an advantage to one treatment over another.