Validity, thus, is the extent to which differences found through a particular data collection procedure reflect true differences among those variables being measured. Reliability, by contrast, refers to the capability to replicate measurements in multiple applications.
To accomplish the objective of validity in measurement, it is necessary to have some standard that is external to the measurement procedure, in order to evaluate the validity of the procedure. An instrument designed to measure some psychological characteristic or factor would be evaluated in the context of validity based on its capacity to yield results consistent with those of another instrument the validity of which previously had been established. Thus, if an instrument were designed to provide a measurement of a tendency toward abnormal behavior, the measurements provided by the instrument would need to be consistent with those of other instruments of proven reliability in the measurement of tendencies toward abnormal behavior.
A measurement procedure is reliable to the extent that it supplies consistent results. In the case of the measurement of tendencies toward abnormal behavior in individuals, an instrument, to be considered as reliable, would be required to yield consistent scores for the same individuals within a short time span when those individuals were both tested and retested with the instrument.
The reliability of a data collection procedure is a contributor to the validity of that procedure, rather than the other way around. An instrument cannot be valid if it is not also reliable; however, just because an instrument is reliable, it is not of necessity valid, because an instrument could yield consistent but inaccurate measurements.
A validity coefficient is the correlation coefficient between a measuring procedure and an outside or independent measure of the function that the test was designed to measure. If those who stand high on a test,...
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