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Friday, August 31, 2012

Finding home field advantage: turnovers

In previous posts I found that home teams in conference games have a .12 point per game advantage in field goal accuracy, a .16 point advantage from penalties, and a .175 point advantage from kickoffs. Total, home teams score 3.15 more points than road teams in these games, so increased field goal accuracy and reduced penalties can account for ~14% of home field advantage.

Turning to turnovers, road teams throw one interception per game on average and home teams throw .93 picks. Roads teams fumble 1.62 times per game and lose 50% while home teams fumble 1.59 times and lose  49%. On average, road teams turn the ball over one extra time every 10 games.

Assigning a point value to that one turnover every 10 games is a bit of a challenge. On average, teams lose by 7 points more/win by 7 fewer points every time they throw an interception, but part of that is because throwing interceptions is related to performance in other areas - e.g., pass protection or desperation at the end of a losing effort. If we adjust that number for the quality of the two teams overall, the point penalty for an interception drops from around 7 to somewhere between 3.5 and 4. For fumbles, the penalty is around 3. Given one extra turnover every 10 games, that would give home teams a .35 point advantage per game.

But we can do better than that. If we assign a point value to field position, down, distance and time remaining in the half, we can directly measure the point impact of a turnover. By that method, fumbles cost 3.49 points on average and interceptions 3.35 points. Given that one turnover advantage every ten games, that comes out to .359 points per game.

To review, of that 3.15 point advantage per game for playing at home, we have accounted for .12 points from increased field goal accuracy, .16 points from fewer penalties, .175 points from better kick coverage and returns, and .36 points for fewer turnovers.   Total, that comes out to .815 points per game or 26% of home field advantage.

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