I agree that distance is not a great metric. The maximum travel distance on a smartphone screen is already tiny. I'd say the best metric is accuracy or lack o

I agree that distance is not a great metric. The maximum travel distance on a smartphone screen is already tiny. I'd say the best metric is accuracy or lack of amibiguity, something like average confidence level that any given swipe means a particular word and not another. (This is assuming swipe-based word entry, which I much prefer to anything tap-based.)