Abstract
An office lettering machine was put through an operability test, and it was discovered that most of the lettering errors seemed to be the result of parallax between a pointer and its scale. A repeat test of the same lettering task with the same machine - but with reduced parallax - showed a 78% drop in the lettering errors. The study demonstrates the effect of machine design on efficiency and productivity, and points to the need to evaluate products in their design stage to optimize the user interface.
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