A big part of the draft are trades. But when trading picks, teams have to find a way to put a value to those picks. To do this, teams use trade value charts. The most widely known of these charts is the "Jimmy Johnson Draft Value Chart" and that version is commonly accepted as the "official" draft value chart.
When Johnson joined the Cowboys in 1989, he took older draft value charts and modified them to his liking, and the end result is the chart we all know. The main criticism of the chart's point value system is that it wasn't created using any real form of statistical analysis, but was based on the available data at the time and research that Johnson and others had gathered previously.
A chart that is much more statistically rigorous in its approach to valuing draft picks is the Harvard Sports Analysis Chart, which was developed by Harvard economics student Kevin Meers and uses a metric called "Career Approximate Value" to value a draft pick. In late 2011, Meers wrote about the specifics of his chart:
"This non-arbitrary statistic is a massive improvement over the old draft chart. The old system massively over-values the earliest picks and significantly undervalues mid-to-late round picks."
Here's what the Harvard Chart looks like:
|Round 1||Round 2||Round 3||Round 4||Round 5||Round 6||Round 7|
Note that this chart shows a linear decline from pick 215 on down, which is different from the original Harvard Chart, which has a distinct and unexplained bump at 215.
After the 2013 draft, the Cowboys were roundly and loudly criticized for giving up value in trading down from the 18th spot for the 31st and 74th pick. (18 = 31 + 74)
The Johnson Chart did indeed suggest that the Cowboys gave up points (900 = 600 + 220) whereas the Harvard Chart (249.2 = 203 + 118.4) indicated that the Cowboys had come out ahead in that trade, a point Stephen Jones stressed in the post-draft press conference:
"There are a whole lot of [trade charts]," Jones said during a press conference Thursday night. "We have one that we work off of and we do update it. I would say that on the ones that we have, for the most part, we either won or hit right on it. This was also a draft where moving back, usually the ones who were moving back were not getting near what they should have had. We felt like we got right on it."
In this day and age, there is no one single "correct" chart, but the odds are that the different charts being used by NFL teams are a lot closer to the Harvard Chart than to the Johnson Chart.