This over-time height measurement is a stability assessment. When babies are born, they are usually 18–24 inches "tall." Parents (and developmental researchers) often measure how tall babies and children are as time passes. In this situation, the researcher need only use either the yardstick or the meterstick using both sticks provides no additional information. When this is so, the use of multiple measures is needlessly repetitive. If two measures of the same concept are perfectly consistent, they provide identical results. This leads to the principle of consistency: For all intents and purposes, the yardstick and the meterstick are interchangeable the researcher need not establish their consistency. With reasonable attention to proper measurement protocol, the correlation between height as measured by the yardstick and height as measured by the meterstick across a sample with sufficient variation in height would be very high. Indeed, the yardstick and the meterstick are highly consistent. Conversely, rounding to two decimal places, if we know that someone is 160 centimeters tall, we also know that that person is 62.40 inches tall (1 centimeter = 0.39 inches 160 × 0.39 = 62.40). For example, rounding to two decimal places, a 70-inch-tall person is 177.80 centimeters tall (1 inch = 2.54 centimeters 70 × 2.54 = 177.80).
If we know how tall someone is in inches, we can calculate how tall he or she is in centimeters, and vice versa. Another common measuring rod is the "meterstick." The meterstick is 100 centimeters long, and is broken down into decimeters, centimeters, and millimeters. 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. In order to measure distance, we establish an arbitrary standard. Height, how tall someone is, is a measure of distance. Let us illustrate consistency and stability on the measurement of height.Īs an example, we often measure how tall people are. Stability is the degree to which a measure of a concept remains unchanged across time stability is based on "longitudinal" research. 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. The reliability of a measured variable has two components, consistency and stability.
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