Evidence for Reliable Individual Differences in Intra-Individual Variability

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Abstract
In reliability theory, the deviations in an individual's performance from his own mean score are referred to as error and the assumption is made that error scores in a series of repeated tests will be uncorrected. To test this assumption, 24 college males were given 175 RT trials a day for 5 days (the first day served as a warm-up session and was not used in the analyses). Within each test day, a short rest period was given following every block of 35 trials. When data were examined on a day-to-day basis, i.e., the total 175 trials per day were used, the reliability coefficients were high (.811 to .866); on a block-to-block within-days basis, i.e., the total 175 trials per day were subdivided into 5 blocks of 35 trials, the coefficients were lower but statistically significant (the average block-to-block correlations for test Days 1, 2, 3 and 4 were .639, .704, .734 and .646 respectively). Re-evaluation of reliability theory and its assumption that an individual's deviations from his own mean ability represent error is needed. Present data suggest these deviations from mean ability in a motor skill actually reflect a biological variability and, as such, should be referred to as intra-individual variability.

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