From the Harvard Sports Analysis website.

Here's the justification for the valuations in the Harvard chart.
Pro-Football-Reference uses a metric called Career Approximate Value (CAV) that allows one to compare players across seasons and positions. It is not meant as the ultimate NFL statistic. It is useful for comparing large groups of players across time and positions, which is exactly the objective here. Using data from 1980 through 2005, I analyzed each overall draft pick from 1 to 224 (the 32nd pick of the 7th round in today’s draft). I found the mean, median, and standard deviation of the CAV for each pick from those 25 years, creating one set of data that represented the historical value of each pick. I then found the mean, median and standard deviation for this new dataset to determine the expected value of a normal draft pick. I then used that normal pick to standardize my data, finding the percentage value over average, or Career Approximate Value Over Average (CAVOA), for every pick in the draft.
And here's the description of the valuations of the JJ chart.
These values are completely arbitrary: there is no statistical evidence to back up the relative values of these draft picks. There is no reason why the 156th pick is 100 times less valuable than the first overall pick. “The Chart” simply dictates how much each pick is worth. These values also have no grounding in the real worth of the players drafted at a given pick. This system is a ridiculous way to value picks because there is no reason behind the values it gives. There must be a better approach.