PROPORTION. A type of ratio in which the numerator is included in the denominator. The ratio of a part to the whole, expressed as a ``decimal fraction'' (e.g., 0.2), as a fraction (1/5), or, loosely, as a percentage (20%).

RATE. An expression of the frequency with which an event occurs in a defined population.

RATIO. The value obtained by dividing one quantity by another.

Source: A Dictionary of Epidemiology, Second Edition, edited by J.M. Last for the International Epidemiological Association and published by Oxford University Press, 1988.