Abortion rate: The annual number of abortions per 1000 women of reproductive age (usually defined as age 15–44 years). For example, in the USA in 1970 the rate was five, in 1980 it was 25 and in 1990 it was 24. [Family Planning Perspectives, 1998, 30, 244–7.]

Abortion ratio: The estimated number of abortions per 1000 live births in a given year. For example, in the USA in 1970 the ratio was 52, in 1980 it was 359 and in 1990 it was 344. [Family Planning Perspectives, 1998, 30, 244–7.]

Abscissa: The horizontal (or x-axis) on a graph, or a particular point on that axis.

Absolute cause-specific risk: Synonym for absolute risk.

Absolute deviation: Synonym for average deviation.

Absolute risk: Often used as a synonym for incidence, although also used occasionally for attributable risk, excess risk or risk difference. Defined more properly as the probability that a disease-free individual will develop a given disease over a specified time interval given current age and individual risk factors, and in the presence of competing risks. Absolute risk is a probability and consequently lies between 0 and 1. See also relative risk. [Kleinbaum, D.G., Kupper, L. L. and Morgenstern, H., 1982, Epidemiologic Research: Principles and Quantitative Methods, Lifetime Learning Publications, Belmont.]

Absolute risk reduction: The proportion of untreated people who experience an adverse event minus the proportion of treated people who experience the event.