Reliability and validity

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Reliability and validity are important concepts within psychometrics.

Reliability is generally thought to be necessary for validity, but it does not guarantee validity. Within classical test theory, predictive or concurrent validity (correlation between the predictor and the predicted) cannot exceed the square root of the correlation between two versions of the same measure — that is, reliability limits validity.

The following relations between reliability and validity may be evident for a particular psychological measure:

  1. Reliable (i.e., consistent) and valid (i.e., measures what its meant to measure)
  2. Reliable (consistent) and not valid (doesn't measure what its meant to measure)
  3. Unreliable (not consistent) and not valid (doesn't measure what its meant to measure)
  4. Unreliable (not consistent) and valid (measures what its meant to measure) - this latter option, however, is conceptually complicated and we need to distinguish between internal reliability and test-retest reliability. A measure of a fluctuating phenomenon such as suicide ideation may be valid but have low test-retest reliability (depending on how much the phenomenon flunctuates and how far apart the test and retest is). In addition, according to classical test theory, internal reliability attenuates validity. But beware of the simplistic mantra that in order for a test be valid, it must reliable. Try to understand and consider the relationship between the two types of reliability (test-retest and internal) and validity.

[edit] Relation between reliability and validity

"Reliability and validity are, conceptually, quite distinct and there need not be any necessary relationship between the two. So be wary about statements which imply that a valid test or measure has to be reliable...In psychology the emphasis in measure is generally on relatively stable and enduring characteristics of people (e.g. their creativity). Such a measure should be consistent over time (reliable). It also ought to distinguish between inventors and the rest of us if it is a valid measure of creativity. A measure of a characteristic which varies quite rapidly over time will not be reliable over time - if it is then we might doubt its validity. For example, a valid measure of suicide intention may not be particularly stable (reliable) over time though good at identifying those at risk of suicide. How reliable it is will depend on the interval between the rest and retest.

Since validity is often expressed as a correlation between the test or measure and some criterion, the validity coefficient (as it is called) will be limited by the reliability of the test or measure. Once again, the maximum correlation of the test of measure with any other variable has an upper limit determined by the internal reliability." - (Howitt & Cramer, 2005)

[edit] Data analysis exercises

[edit] See also

[edit] References

Howitt, D. & Cramer, D. (2005). Reliability and validity: Evaluating the value of tests and measures (Ch. 13). In Introduction to research methods in psychology (pp. 218-231). Harlow, Essex: Pearson.

[edit] External links

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