Response probability curves under four conditions of task length were skewed and S-shaped. Each curve originated at a value consistent with the initial chance solubility of the problem. The inflection points were a direct function of task length and the acquisition rates varied inversely. Errors were independent of serial position but found permutation by position interactions. Discussion is in terms of Thurstone's length-difficulty function and the Kjerstad-Robinson law. (PsycINFO Database Record (c) 2006 APA, all rights reserved)