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Wechsler, D. (2002). Wechsler Intelligence Scale for Children, third edition. Found online at: http://ericae.net/tc3/TC017905.htm
Reliability refers to the accuracy and precision of a of data collection procedure and validity refers to the extent to which data or a data collection instrument measures what it is actually desired to measure (Gravetter & Wallnau, 2000). 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. Thus, if the objective were to measure the height of a group of elementary school students in terms of feet and inches, it would be necessary to have a measuring instrument calibrated in feet and inches. The basis for assessing the validity of this measuring instrument calibrated in feet and inches, would be its ability to accurately measure feet and inches in accordance with an external standard, such as a master measurement instrument which is maintained by the National Bureau of Standards. Evaluation of the validity of an instrument designed to measure some psychological characteristic or factor, would rely on its ability to yield results consistent with those of another instrument of established validity. Thus, to be valid, data provided by an instrument designed to measure a tendency toward abnormal behavior, would have to be consistent with those of other instruments of proven reliability in the measurement of tendencies toward abnormal behavior (Lockhart, 1998).
The choice between probability levels of Type I (Alpha) and Type II (Beta) errors of measurement is the basis for assessing the relative importance of two alternative types of mistakes in hypothesis testing in statistical inference analysis. In such analysis, decision procedures rest entirely on the analysis of data collected through a random sampling of the total population (Yates, Moore, & Mc