Memory & Cognition

, Volume 23, Issue 6, pp 689–700

Why is 9+7 harder than 2+3? Strength and interference as explanations of the problem-size effect


DOI: 10.3758/BF03200922

Cite this article as:
Zbrodoff, N.J. Mem Cogn (1995) 23: 689. doi:10.3758/BF03200922


In four experiments, the problem-size effect was investigated, using an alphabet-arithmetic task in which subjects verified such problems as A + 2 = C. Problem size was manipulated by varying the magnitude of the digit addend (e.g., A + 2, A + 3, and A + 4). The frequency and similarity of problems was also manipulated to determine the contribution of strength and interference, respectively. Experiment 1 manipulated frequency at low levels of practice and found that strength could account for the problem-size effect. Experiment 2 manipulated frequency at higher levels of practice, and found that strength alone could not account for the problem-size effect at asymptote. Experiment 3 manipulated frequency and similarity and found a substantial problem-size effect at asymptote, suggesting that both strength and interference contribute to the problem-size effect. Experiment 4 manipulated similarity, keeping frequency constant, and found no problem-size effect at asymptote, suggesting that interference alone is not responsible for the problem-size effect. The results are related to findings with number arithmetic.

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Copyright information

© Psychonomic Society, Inc. 1995

Authors and Affiliations

  1. 1.Department of PsychologyUniversity of IllinoisChampaign

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