by Max Mulitz
Theory of Run Stops Added:
A run stop is a play where a defender makes a tackle on a running play that is not efficient for the offense. Rushing Success Rate correlates with winning better than Yards Per Carry, so the measure of a run defenders value can be measured by his ability to prevent successful runs. By determining the value of a run stop and measuring the players’ ability to generate run stops, we can solve for the players’ value as a run defender.
How Valuable is a Run Stop? A run stop is worth about 1.3 points. Runs that are unsuccessful have an expected point value of about 0.75 expected points, while unsuccessful runs have an expected points value of about -0.55 expected points, so a run stop is worth about 1.3 expected points on average. Of course, a 5 yard loss on 3rd & 1 at the Goal Line has a greater impact than a tackle for no gain on 1st & 10 at midfield, but in aggregate a run stop is worth about 1.3 points.
How does an individual player contribute to his team making Run Stops? Different positions have different base run stop percentages. A replacement level 4-3 defensive end may only make a run stop on 6% of his run plays while a replacement level MLB may make a run stop on closer to 7.5% of his plays. Players in different schemes will have different opportunity levels, but a middle linebacker on the Raiders is has a more similar job to a middle linebacker on the Chiefs in terms of run defense than he will have to a defensive tackle on his own team.
Modeling Individual Run Defense: A players’ contribution to stopping the run can be estimated as the percentage of run plays where he generates a run stop minus the expected run stop percentage for his position, multiplied by the number of snaps they played in a given season, as seen in the formula below:
Run Stops Added=(Run Stop %-Position Replacement Level Run Stop %)*Number of Snaps
For instance, a Defensive End who played 200 Run Defense Snaps and had a 8% Run Stop Rate would be responsible for 4 extra run stops above a 6% replacement level player. At 1.3 points per run stop, the players contribution could be estimated at 5.2 points (about 0.14 wins.)
Run stops added is a reasonably consistent statistic from year to year, which is a necessity for it to be used as an indicator of defensive player value. For example, for Defensive Ends, Run Stops Added for players with over 200 run snaps in both years were strongly correlated from 2014 to 2015 at R=.66.
Limitations of Run Stops Added:
- Certain players may improve their teams’ run defense statistics by making their teammates’ job easier. If a star defensive tackle demands a double team, thereby allowing the teams middle linebacker to often go unblocked and make more plays in the run game, the tackles contribution will reflect in the linebackers statistics, but not his own. Dominant defensive linemen/run stopping LBs are usually able to create for themselves often enough that we are unlikely to have many “No Stats All Stars” in primary run defense, though the stats obviously only tell most of the story. In general, the players who are considered top run defenders are able to translate that into a high run stop percentage.
- Scheme obviously matters. Some teams schemes are going to enable their weak side linebackers to make relatively more tackles in the run game than other teams irrespective of individual player ability.
- Run Stop Percentage is not appropriate for secondary run defenders, as it does not reward tackles in the open field (after the run has already “succeeded” in terms of improving first down probability.)
Damon Harrison is a good check on the calibration of our model. Harrison is perhaps the best run stopper in the NFL, though he brings nothing to the table as a pass rusher. In 2015, Harrison’s Run Stops Added would have valued him at almost exactly one win above replacement. With a 143 Million Dollar Salary Cap in 2016, this would imply an open market value of 9.53 million. In fact, Harrison signed with the Giants for an average of 9.25 million per season.
Why Run Stops Added instead of other metrics:
Tackles for loss are useful, but because tackles for loss are a subset of run stops, there is always a smaller number of TFLs than Run Stops in a given year, making them less stable over a sample of only a few hundred snaps.
Percentage Share of Run Game Tackles/Total Tackles can be useful, but doesn’t tell us if a player is making a lot of tackles because he’s making plays in the run game or because he’s chasing players down at the second level after they beat his teammates. An average run defender surrounded by replacement level teammates is going to have the tackle totals of a stud, for instance.
Expected Points Added is useful, but may overweight plays in certain situations. For instance, a stop on 4th & Goal at the 1 could be more valuable than five run stops over the course of a game in the middle of the field. In the long run, the player who made five stops in the run game has shown more of a repeatable skill set than the player who happened to make only one play at exactly the right time.
Run Stop Percentage and Run Stops Added are the fundamental metrics of a defenders’ contribution in the run game. A players Run Stops Added can be converted to expected points added and therefore wins added, which serves as a starting point to value an individual player’s contribution as a run defender. Run Stops Added can also be used for advanced scouting purposes to help discern who the opponents’ best and worst run defenders are.