People don't know just what to do with the metaphor of the learning curve, speaking of it in such a variety of terms:
|Type of learning curve||Hits in Google (13 June 2007)|
I think everyone agrees that the horizontal coordinate is time, but the confusion is whether the vertical coordinate is effort or results. The Wikipedia article suggests that the original sense had to do with the outcome of the learning rather than the difficulty of it, which seems to be the more prevalent interpretation these days. My idea is that anyone throwing around the metaphor ought to provide an actual picture of the curve they have in mind, with labels along the axes, so that the person hearing the phrase knows which sense is intended without having to resort to context.