The accompanying article boldly claimed that
the 40-yard dash stands above them all as having the strongest correlation to success in the NFL. The three-cone drill, the shuttle run, the bench press -- none correlate to NFL success. The 40 is king.
Further, it cited Bill Barnwell from FootballOutsiders.com who created an "index" using both 40 time and body weight that is "an even better predictor than 40 time alone". In other words, this formula
does the trick.
The data table, shown above, presumably clinched the case.
We were mystified when we put the data to the test, however. Among the set of 15 running backs, the Index did not predict the Yards Per Carry at all! The Index explained only 8% of the variation in Yards Per Carry between the backs.
The data table obscures this bivariate relationship. As it was sorted by the Index, we would look for the column showing Yards Per Carry to be naturally sorted in the same order. But it is hard to tell the trend from the noise in a table.
What went wrong? It turned out neither 40 Time nor Body Weight had any relationship with Yards Per Carry.
These variables did not explain the range of Yards Per Carry attained by this set of running backs.
Finally, we found strong correlation between 40 Time and Body Weight. (The heavier you are, the slower you run!) This meant that both variables contained similar information and some unlikely formula involving the two would be unlikely to perform significantly better than each variable alone.
So we are left to turn the table on the table. More pertinent evidence is needed to prove the case.
The entire analysis suffers from survivorship bias as only the top
running backs are examined, and no adjustment is made to deal with
wide-ranging tenures. Apparently, there is more data available in a book. There is no indication of how the model shown above was validated.
Reference: "The Race of Truth: 40-Yard Times Can Tell the Future", New York Times, April 27, 2008.