The “50%” point is the median. So, the point at which it is just as likely to have happened as to not have happened. The mean will in general be different from

The “50%” point is the median. So, the point at which it is just as likely to have happened as to not have happened. The mean will in general be different from the median if the distribution is skewed. Here the mean is pulled a little higher than the median by a few years with a “long tail” of estimates in the later years. The distribution can be seen in figure 4(b)