Over the years, Bill James came up with a ton of innovative ways to gauge and analyze player and team performance, but he recognized that there was one uncontrollable factor that likely would always escape any attempts to gauge it: luck. At one point, James estimated that luck could account for 30 percent of what happens on a baseball field.
This rule almost certainly applies to the world of pro football, too. A good example of this can be found in the supposed improvement of the Chicago Bears' defense this year.
While there is no doubt this unit has made progress in some of its statistics, after taking a closer look at the numbers, it became clear that good fortune has as much to do with this group's improvement as anything.
To help illustrate the specifics behind the meaning of that last sentence, let's do a closer inspection of Chicago's interceptions.