There is uncertainty about the terms used to describe the properties of measurement: accuracy, precision, validity, reliability, repeatability, and reproducibility. Accuracy and precision are often used synonymously, validity is defined variously, and reliability, repeatability, and reproducibility are frequently used interchangeably. Etymologies are helpful in making a case for preferred usages, but they are not always decisive. Accuracy is from the Latin cure, care, and while this may be of interest to those in the health field, it does not illuminate the origins of the standard definition, that is, "conforming to a standard or a true value" (OLD). Accuracy is distinguished from precision in this way: A measurement or statement can reflect or represent a true value without detail. A temperature reading of 98.6ºF is accurate, but it is not precise if a more refined thermometer registers a temperature of 98.637ºF. Precision, (from Latin praecidere, cut short) is the quality of being sharply defined through exact detail. A faulty measurement may be expressed precisely, but may not be accurate. Measurements should be both accurate and precise, but the two terms are not synonymous. In Kendall and Buckland's Dictionary of Statistical Terms, precision is defined as "a quality associated with a class of measurements and refers to the way in which repeated observations conform to themselves." Standard, as well as epidemiologic definitions are not in agreement with this statistical usage. Consistency or reliability describes the property of measurements or results that conform to themselves. Reliability (Latin religare, to bind) is defined by the OED as a quality that is sound and dependable. Its epidemiologic usage is similar; a result or measurement is said to be reliable when it is stable, i.e., when repetition of an experiment or measurement gives the same results. The terms "repeatability" and "reproducibility" are synonymous (the OED defines each in terms of the other), but they do not refer to a quality of measurement, rather only to the action of performing something more than once. Thus, a way of discovering whether or not a measurement is reliable is to repeat or reproduce it. The terms "repeatability" and "reproducibility," formed from their respective verbs, are used inaccurately when they are substituted for "reliability," a noun that refers to the measuring procedure rather than the attribute being measured. However, in common usage, both repeatability and reproducibility refer to the capacity of a measuring procedure to produce the same result on each occasion in a series of procedures conducted under identical conditions. Validity is used correctly when it agrees with the standard definition given by the QED: "sound and sufficient." If, in the epidemiologic sense, a test measures what it purports to measure (it is sufficient) then the test is said to be valid. [Last, 1983: A Dictionary of Epidemiology]