Calculation latency: the mu of memory and the tau of transformation

Does numeral format (e.g., 4 + 8 vs. four + eight) affect calculation per se? University students (N = 47) solved single-digit addition problems presented as Arabic digits or English words and reported their strategies (memory retrieval or procedures such as counting or transformation). Decomposition of the response time (RT) distributions into mu (reflecting shift) and tau (reflecting skew) confirmed that retrieval trials contributed predominantly to mu whereas procedure trials contributed predominantly to tau. The format x problem size RT interaction (i.e., greater word-format RT costs for large problems than for small problems)was associated entirely with mu and not with tau. Reported use of procedures presented a corresponding format x size interaction. Together, these results indicate that, relative to the well-practiced digit format, the unfamiliar word format disrupts number-fact retrieval and promotes use of procedural strategies.