But the truth is told by the third graph. First of all, the trend that easiness rises with quality is a lie. As people can see from this graph, the trend is nearly flat. The reason is I used the rank of professors in their college as the x-axis in previous graphs, but I used the real rate of quality for each professor as the x-axis now. Furthermore, I did not include the Math Professors' line which is really flat. The rest of data can 'support' my words.

Also, professors I picked are those that are popular (most rates), which to some extents are the best in students' mind. As shown in the graph, the best professor in Math and SCS have highest quality, while science and econ professors have comparatively lower quality. (This does not aim at any certain professor or college, but only the analysis from the data)