Memory can be internal or external - knowledge in-the-world or knowledge in-the-head. Making needed information available in an interface may seem the perfect alternative to relying on imperfect memory. However, the rational analysis framework (Anderson, 1990) suggests that least-effort tradeoffs may lead to imperfect performance even when perfect knowledge in-the-world is readily available. The implications of rational analysis for interactive behavior are investigated in two experiments. In experiment 1 we varied the perceptual-motor effort of accessing knowledge in-the-world as well as the cognitive effort of retrieving items from memory. In experiment 2 we replicated one of the experiment 1 conditions to collect eye movement data. The results suggest that milliseconds matter. Least-effort tradeoffs are adopted even when the absolute difference in effort between a perceptual-motor versus a memory strategy is small, and even when adopting a memory strategy results in a higher error rate and lower performance.
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