Memory & Cognition 23(6), 689-700.

**ISSN/ISBN:** 0090-502X
**DOI:** 10.3758/BF03200922

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**Abstract:** 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.

**Bibtex:**

```
@article{,
title={Why is 9+ 7 harder than 2+ 3? Strength and interference as explanations of the problem-size effect},
author={Zbrodoff, N Jane},
journal={Memory \& Cognition},
volume={23},
number={6},
pages={689--700},
year={1995},
publisher={Springer},
ISSN={0090-502X},
DOI={10.3758/BF03200922},
}
```

**Reference Type:** Journal Article

**Subject Area(s):** Psychology