A

A priori analysis: See planned analysis.

Absolute risk difference: See risk difference.

Absolute risk increase (ARI): See risk difference.

Absolute risk reduction (ARR): See risk difference.

Activities of daily living (ADL): See functional status.

Adherence: The consistency and accuracy with which a patient follows a recommended medical regimen.1 Also called compliance. See also persistence.

Adverse effect: A harmful or undesirable outcome occurring during or after use of a drug or intervention where there is a reasonable possibility of a causal relation.2

Adverse event: A harmful or undesirable outcome occurring during or after use of a drug or intervention but not necessarily caused by it.2

Adverse reaction/ adverse drug reaction: An adverse effect specifically associated with a drug.2

Agency for Healthcare Research and Quality (AHRQ): The lead federal agency charged with improving the quality, safety, efficiency, and effectiveness of health care for all Americans.3 As one of 12 agencies within the Department of Health and Human Services, AHRQ supports health services research to improve health care quality and promote evidence-based decision-making. See also Effective Health Care Program. Website: http://www.ahrq.gov.

AHRQ: See Agency for Healthcare Research and Quality.

Alpha error: See Type I error.

Alternative hypothesis: The opposite of the null hypothesis.4 It is the conclusion when the null hypothesis is rejected.

Applicability: The extent to which the effects observed in published studies are likely achieve similar results when the same intervention is applied to the population of interest under “real-world” conditions (i.e., typical practice). Also called external validity, generalizability.5

Association: A relationship between two variables (characteristics), such that as one changes, the other changes in a predictable way.6 A positive association occurs when one variable increases as another one increases. A negative association occurs when one variable increases as the other variable decreases.6 Association does not imply causation.7 Also called correlation.

Attrition: Loss of participants during the course of a study.6 Participants lost during the course of a study are often called dropouts. Also called lost to follow up.

Attrition bias: Systematic differences between comparison groups in withdrawals or exclusions of participants from the results of a study.6 For example, participants may drop out of a trial because of side effects of the intervention. Excluding these participants from the analysis could result in an overestimate of the effectiveness of the intervention or an underestimate of side effect rates, especially when the proportion dropping out varies by treatment group.