Absolute Risk Reduction1

The absolute difference in rates of an outcome between treatment and control groups in a clinical trial.

For example, a clinical trial compares the effect of a new statin and placebo on the incidence of stroke. Over the course of the study, the incidence of stroke is 4% with the statin and 6% with placebo. The absolute risk reduction with the statin is 2%.


A way of making sure that the people involved in a research study - participants, clinicians, or researchers - do not know which participants are assigned to each study group. Blinding is used to make sure that knowing the type of treatment does not affect a participant's response to the treatment, a health care provider's behavior, or assessment of the treatment effects.

Confidence Interval (CI)1

Given the results of a study, the confidence interval is a range given around that study’s results where we expect that the true value will be present.


A factor in a study that affects the results of the study but was not a variable of interest.

Cost-Benefit Analysis (CBA)2

A type of analysis that compares the monetary cost with the benefits of two or more health care treatments or programs. Health care interventions that have the same or better benefit at a lower cost are of better value than treatments or programs that are more expensive.

For example, cost-benefit analyses have been conducted to compare vaccinating people against a certain disease versus treating those people who get sick from the disease when no one is vaccinated.

Cost-Effectiveness Analysis (CEA)2

A type of analysis that is similar to a cost-benefit analysis but is used when the benefits cannot be measured in financial terms or dollars. It would be hard to put a price-tag on living an extra year of life.

For example, a cost-effectiveness analysis might compare the costs of two health care interventions that both helped people to live an extra year.

Crossover Study3

In this study design, each patient receives both treatments. For example, a sample of patients receive treatment A at first. After some time, at a crossover point, they start treatment B.

There is less variability in outcomes because the patient serves a his/her own control. Reduced variability means a smaller sample size is needed than for a parallel-group trial. The two phases of the study are usually separated by a washout period. Crossover studies are susceptible to period effects - differences in the effectiveness of a drug due to the passage of time. Period effects can be attributed to the development of tolerance or resistance, learning effects, or changes in the course of the disease being treated. Therefore, this study design is usually reserved for investigations where the outcome is reversible with time.


Also referred to as the outcome measure or the clinical endpoint, this refers to the outcome or event that is used to measure the efficacy of a drug or other intervention in a clinical trial. Endpoints include, toxicity, relief of symptoms, and improvements in quality of life.

Evidence-Based Practice5

Is the conscientious, explicit, and judicious application of the best available research results (evidence) when making decisions about the care of individual patients. Health care professionals who perform evidence-based practice means integrating individual clinical expertise with the best available external clinical evidence from systematic research. Systematic reviews (summaries of health care research results) provide information that aids in the process of evidence-based practice.

Health Technology Assessment6

A method that determines the value of an intervention at different points in its lifecycle in order to inform on decisions that promote an equitable, efficient, and high-quality health system.


In a meta-analysis or systematic review, when the results of individual studies are compatible with one another they are considered to be homogenous. Heterogeneity is the amount of incompatibility between studies in the review, which can be caused by clinical or statistical differences.


A way of combining data from many different research studies. A meta-analysis is a statistical process that combines the findings from individual studies into a single estimate of effect.

For example, researchers wanted to know about the risk of stomach bleeding in people taking aspirin. They did a meta-analysis of data from 24 clinical trials with nearly 66,000 participants and found that the risk of stomach bleeding was 2.47 percent with aspirin compared to 1.42 percent with placebo (inactive substance).

Number Needed to Harm (NNH)1

The number of patients treated with a specific therapy in order for one of them to have a bad outcome.

Number Needed to Treat (NNT)1

The number of patients needed to treat with a specified therapy in order for one patient to benefit from treatment. The NNT is the inverse of the absolute risk reduction (1/absolute risk reduction).

Odds Ratio (OR)6

The ratio of the exposure odds among cases to the exposure odds among the control.

Odds ratios and relative risk are comparable when the outcome is rare. But the odds ratio can make risk appear greater when the disease or outcome is more common. In case-control studies evaluating the risk of an adverse effect, an odds ratio of 1 indicates that exposure to the drug is equally likely in cases and controls. If the odds ratio is greater than 1, the risk of exposure is greater in cases than controls. If the odds ratio is less than 1, the risk of exposure is smaller in cases than controls.

Open-Label Study4

A clinical research study in which the participant, health care professional, and others know the drug and dose being given. The research study is not "blinded."


The probability that a particular result would have occurred merely by chance.

Placebo Controlled Study4

A type of clinical trial in which the effect of an active drug is compared with the effect of a placebo (an inactive substance designed to resemble the drug). In placebo controlled clinical trials, participants receive either the drug being studied or a placebo. The results of the drug and placebo groups are then compared to see if the drug is more effective in treating the condition than the placebo is.

Placebo Effect4

A physical or emotional change that occurs after a participant in a research study takes a placebo. The change, which may include the lessening of symptoms, is not the result of chemical effects of the placebo because the placebo does not contain any active ingredients. The change is often based on the participant's or researcher's expectation that a change will occur.

In a large research study of people with arthritis of the knee, one group of participants took a placebo pill. Sixty percent of these people reported improvement in their pain and functioning while taking the pill. This clinical improvement was considered to be a placebo effect.

Positive Predictive Value1

Proportion of people with a positive diagnostic test who have the disease.

Randomized Controlled Trial (RCT)4

A prospective study in which patients are assigned by chance (randomized) into treatment or control groups. These groups are followed up for the variables/outcomes of interest.

Relative Risk (RR)1

The risk of an event in individuals with a particular characteristic compared with the risk of that event in individuals who don't have that characteristic. This is the probability of an event in the treatment group divided by the probability of that event in the placebo group.


The ability of a test to reliably detect the presence of a disease. The proportion of patients with the disease who have a positive test. Sensitivity = 100 % true negatives/true negatives + false positives.


The ability of a diagnostic test to reliably rule out a disease. The proportion of patients without the target disease who have a negative test. Specificity = 100 % true negatives/true negatives + false positives.

Systematic Review1

A summary of the clinical literature. A systematic review is a critical assessment and evaluation of all research studies that address a particular clinical issue. The researchers use an organized method of locating, assembling, and evaluating a body of literature on a particular topic using a set of specific criteria. A systematic review typically includes a description of the findings of the collection of research studies. The systematic review may also include a quantitative pooling of data, called a meta-analysis.


The soundness of rigour of a study. A study is internally valid if the way it is designed and carried out means that the results are unbiased and it gives you an accurate estimate of the effect that is being measured. A study is externally valid if its results are applicable to people encountered in regular clinical practice.


  1. Law K, Howick J. Glossary. Centre for Evidence-Based Medicine, University of Oxford. [cited 12 NOV 2020]. Available from:
  2. British Medical Journal. A glossary of health economics terms. BMJ Best Practice. [cited 12 NOV 2020]. Available from:
  3. Centre for Evidence-Based Medicine. Study designs. Centre for Evidence-Based Medicine, University of Oxford. [cited 12 NOV 2020]. Available from:
  4. US Department of Health and Human Services. AIDS Info Glossary of HIV/AIDS Related Terms. 2018. [cited 12 NOV 2020]. Available from:
  5. Sackett DL, Rosenberg WM, Gray JM, Haynes RB, Richardson WS. Evidence based medicine: what it is and what it isn't. BMJ 1996;312(7023):71–72. Doi: 10.1136/bmj.312.7023.71
  6. Facey K, Topfer LA, Chan L. Health technology assessment (HTA) glossary. INAHTA–International Network of Agencies for Health Technology Assessment. 2006. Available from: