To Miss is Human

Abstract

In usual Fitts' law experiments the outcome of a pointing act can be either measured as an error, i.e., a distance from endpoint to target center, or categorized in an all-or-none way as a hit versus a miss. Information theory offers a useful distinction between transmission errors (the received symbol is wrong) and erasures (the received symbol is empty). Although Fitts' law research has been very much inspired by the information theoretic rationale, the error/erasure distinction has escaped attention so far: Target misses have always been treated as normally-distributed errors, through the effective index of difficulty IDe. The paper introduces a new index of difficulty based on the simple observation that a target miss conveys zero bit of information, i.e., it is an erasure. Not only is the new index more consistent with the fundamentals of information theory, it is much simpler to derive than the ISO-recommended IDe.