This article examines the reliabilities (interday correlations) across 15 days of repeated measurements for each of five prominent human information-processing tasks: proactive memory interference, semantic reasoning, letter search, typographic error search, and choice reaction time. In each case, the reliability of the slope scores is less than the reliabilities of the mean response times from which the slopes were calculated. Reasons for the relative unreliability of slope scores are discussed. Finally, it is argued that in applied research it is usually unnecessary to calculate slope scores for individuals because the more reliable mean response times are sufficient to answer common applied questions.
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