The reliability of a measured variable has two components, consistency and stability. Consistency is the degree to which two measures of the same concept provide the same assessment at the same time; consistency is based on "cross-sectional" research. Stability is the degree to which a measure of a concept remains unchanged across time; stability is based on "longitudinal" research. Let us illustrate consistency and stability on the measurement of height.

The Measurement of Height

As an example, we often measure how tall people are. Height, how tall someone is, is a measure of distance. In order to measure distance, we establish an arbitrary standard. A common arbitrary standard for measuring distance is the "yardstick." The yardstick is 36 inches long, and is broken down into feet, inches, and fractions of inches. Another common measuring rod is the "meterstick." The meterstick is 100 centimeters long, and is broken down into decimeters...