Q: Should fumbles be treated differently from interceptions?

A: It's hard to refute the significance of turnovers to which team covers the spread, both in terms of the "turnover difference" theory projecting which team is the better bet (56.6% over 18 years), and in terms of how often the team with fewer turnovers in the game itself covers (75% over a five year span).

However, many people have argued that while fumbles may involve a certain amount of luck, interceptions are more dependent on the skill of the players and schemes involved. So our first tack will be to examine each factor individually, starting with fumbles, in our T/O Difference angle -- look for matchups where the net fumbles per game difference between the teams is at least 0.5 or more and play the team with the WORSE fumble record.(e.g. Team A is -1.0 net fumbles per game, Team B is +0.5, so difference between them is 1.5 and play the worse team, Team A).

So the overall record against the spread using only fumbles and discounting interceptions is a little worse than when we use both. The underdog plays hit a solid 55.5%, but that rises to 57.5% in games where the underdog is getting 5+ points -- which is very similar to the full (fumbles and interceptions) T/O results.

2001 UPDATE:
The Net Fumble turnover difference plays posted an exceptional record of 57-35 (62%) from week five on last season. Certainly the rash of dogs covering helped the record, but Fumbles will continue to be a misunderstood stat by the public, linemakers, and NFL gamblers throughout the world.

Let's take a look then at Interceptions alone:

ANALYSIS: The overall results drop a little further: 54.0% for Interceptions alone, 55.0% for fumbles alone, and 56.6% using a combined net turnover number.

However, some interesting things happen with the "interceptions only" approach, namely that the bigger the difference the more successful the plays have been -- requiring at least a 1.0 net difference (instead of the customary 0.5) between the two sides in a matchup yields a healthy 375-289 record (56.4%), and 170-121 (58.4%) if you can get 1.5 or more.

So in conclusion it appears that both fumbles and turnovers have independent predictive value, although the turnover difference theory is strongest overall when you use both. A fumble is arguably more significant (or more damaging to a team), but the interceptions are worth following alone for instances of wide variance between two squads.

2001 UPDATE:
Interception turnover difference plays were likewise solid as a rock in 2001, compiling a nifty 65-44 (60%) mark from week five on. Interestingly, the truly big difference situations (1.5+ net INT per game difference) were just 7-7, suggesting that too much slicing and dicing is not beneficial -- stick with the basic tenets and prosper!