 Discriminant validity

Discriminant validity describes the degree to which the operationalization is not similar to (diverges from) other operationalizations that it theoretically should not be similar to.
Campbell and Fiske (1959) introduced the concept of discriminant validity within their discussion on evaluating test validity. They stressed the importance of using both discriminant and convergent validation techniques when assessing new tests. A successful evaluation of discriminant validity shows that a test of a concept is not highly correlated with other tests designed to measure theoretically different concepts.
In showing that two scales do not correlate, it is necessary to correct for attenuation in the correlation due to measurement error. It is possible to calculate the extent to which the two scales overlap by using the following formula where r_{xy} is correlation between x and y, r_{xx} is the reliability of x, and r_{yy} is the reliability of y:
Although there is no standard value for discriminant validity, a result less than .85 tells us that discriminant validity likely exists between the two scales. A result greater than .85, however, tells us that the two constructs overlap greatly and they are likely measuring the same thing. Therefore, we cannot claim discriminant validity between them.
Consider researchers developing a new scale designed to measure Narcissism. They may want to show discriminant validity with a scale measuring Selfesteem. Narcissism and Selfesteem are theoretically different concepts, and therefore it is important that the researchers show that their new scale measures Narcissism and not simply Selfesteem.
First, we can calculate the Average InterItem Correlations within and between the two scales:
 Narcissism — Narcissism: 0.47
 Narcissism — Selfesteem: 0.30
 Selfesteem — Selfesteem: 0.52
We then use the correction for attenuation formula:
Since 0.607 is less than 0.85, we can conclude that discriminant validity exists between the scale measuring narcissism and the scale measuring selfesteem. The two scales measure theoretically different constructs.
See also
References
 Campell, D. T., & Fiske, D. W. (1959). Convergent and discriminant validation by the multitraitmultimethod matrix. Psychological Bulletin, 56, 81105.
 John, O.P., & BenetMartinez, V. (2000). Measurement: Reliability, construct validation, and scale construction. In H. T. Reis & C. M. Judd (Eds.), Handbook of research methods in social psychology, pp. 339369. New York: Cambridge University Press.
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