Learning Curve

Many people use the phrase "steep learning curve" to refer to something that is difficult to learn. This makes sense if one thinks of plotting amount to be learned vs. time. If one plots amount learned vs. time, then a steep (or short) learning curve is a good thing, because it indicates that a large amount of learning is taking place in a short period of time. In contrast, a shallow (or long) learning curve indicates that a long period of time is required to learn the needed information.