Thus far, I have not defined the crucial concept of this book—the concept of method. What is a psychological assessment method? It is a set containing a variety of instruments and procedures that uncover psychological attributes of objects and transform these attributes into symbols that can be processed. "Psychological attribute," "object," "symbol," and "processing" are themselves sets that contain a variety of elements. Typical attributes are personality attributes, typical objects are individuals, typical symbols are numbers, and computing a mean is a typical way of processing symbols. This book gives an overview of the large variety of assessment methods constructed in the history of scientific psychology.

Good assessment methods are objective, reliable, and valid (Anastasi, 1988; Cronbach, 1960; Wiggins, 1973). Objectivity means that results do not depend on who administered or scored the instrument. Reliability means that results can be replicated under the same conditions. Validity means that the method measures what it is supposed to measure. Regarding each criteria, the quality of an assessment method can be defined as a special type of consistency or convergence. Objectivity can be defined as the amount of convergence across researchers or practitioners who use the method. Reliability can be defined as the amount of convergence across repeated applications of the method for the same objects under the same conditions. Construct validity can be defined as the amount of convergence of the measured (manifest) attribute with the true (latent) attribute or construct (Shadish et al., 2002). Because constructs are hypothetical and cannot be observed directly, they must be substituted either by another measure or some criterion (Cronbach & Meehl, 1955). Although broader and more complex definitions of construct validity have been offered and although multifold procedures for establishing construct validity have been proposed (Messick, 1989), all conceptualizations of construct validity eventually result in the notion of convergence between the measured attribute and the "true" attribute as it appears in theoretical statements about the phenomenon to be described and explained.

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