How Defensive Backs Impact Starting Position via Interception Yardage, or SPI%

The previous post discussed Possessions Responsible for Ending, Possessions Ended, and, focally, Possession Recovered. To accompany that, I planned a metric tentatively termed Starting Position Impact, or SPI. Exactly as the term indicates, SPI is designed to account for a DB’s impact on his team’s average starting field position on offense via his defense of the passing game, currently via interception return yardage but, in the future, ideally, also via pass deflections.

Because drive data is readily available for professional teams, I compiled a data set for all NFL teams from regular seasons 2010-11 through 2014-15. I gathered data for 713 player-seasons in that time period for players who, in a given season, suited up for at least four games and netted at least 1 interception. Players who played for multiple teams within a given season were assigned League average Offensive Drive and Average Drive Starting Position values for that season.

IYPOD is Interception Yards Per Offensive Drives. ADSP is Average Drive Starting Position (on offense).

SPI is calculated simply with the above sequence of equations. Table 1 contains the necessary data for the top-35 DBs in SPI since 2010. The top-35, incidentally, are the DBs who are estimated to have increased their teams’ Starting Position by 2% or greater. That is, without these DBs yardage contributions on INT returns, their teams would have started drives that percentage nearer to their own endzone. 

For perspective, I also included in Table 1 Ed Reed (2004) and Darren Sharper (2009) for reference, the top-2 historically in single season INT yards. Likewise, I included IYPOD estimates for seasons from Deion Sanders, Charlie McNeil, and Night Train Lane. For those three, Team Drives were estimated by summing TDs, FGAs, Punts, game kickoffs, and Offensive TOs, and, for Sanders, failed 4^th-down conversion attempts as well.

However, as the regular reader has probably come to realize…I sought a more nuanced and explicatory metric supplementary to SPI. Before I explicate that nuanced metric, however, I will devote the next post to examining how team play on defense and special teams impacts starting field position on offense.

Table 1. Defensive Backs who Increased Their Team's Average Starting Field Position by >2% via INT Yards

Rank	PLAYER	Tm	Year	INT Yds	Drives	Drive Start	Yds/Drives	SPI%
1	Stevie Brown	NYG	2012	307	176	31.4	1.74	5.56
2	Chris Houston	DET	2011	225	197	28.7	1.14	3.98
3	Brandon Browner	SEA	2011	220	192	33.5	1.15	3.42
4	Janoris Jenkins	STL	2012	150	182	24.4	0.82	3.38
5	Ed Reed	BAL	2010	183	189	29.8	0.97	3.25
6	Harrison Smith	MIN	2014	150	176	27.3	0.85	3.12
7	Kerry Rhodes	ARI	2010	174	190	29.7	0.92	3.08
8	Tashaun Gipson	CLE	2014	158	193	26.8	0.82	3.05
9	Rashad Johnson	ARI	2014	146	181	26.9	0.81	3.00
10	Darrelle Revis	NYJ	2011	184	203	30.9	0.91	2.93
11	Ronde Barber	TAM	2012	160	193	29.0	0.83	2.86
12	Tashaun Gipson	CLE	2013	143	195	26.4	0.73	2.78
13	Charlie Peprah	GNB	2011	147	180	30.8	0.82	2.65
14	Chris Harris Jr	DEN	2012	144	188	29.1	0.77	2.63
15	Eric Reid	SFO	2014	138	175	30.5	0.79	2.59
16	Captain Munnerlyn	CAR	2012	107	163	25.5	0.66	2.57
17	Kendrick Lewis	KAN	2011	119	189	24.6	0.63	2.56
18	William Gay	PIT	2014	113	174	25.7	0.65	2.53
19	Matt Giordano	OAK	2011	130	193	27.2	0.67	2.48
20	Brandon Carr	DAL	2012	120	180	27.2	0.67	2.45
21	Brandon Boykin	PHI	2013	136	201	27.9	0.68	2.43
22	Janoris Jenkins	STL	2014	124	186	28.0	0.67	2.38
23	Casey Hayward	GNB	2014	113	169	28.5	0.67	2.35
24	Darius Butler	IND	2012	101	180	24.8	0.56	2.26
25	A.J. Bouye	HOU	2014	120	194	27.7	0.62	2.23
26	Glover Quin	DET	2014	117	179	29.9	0.65	2.19
27	Asante Samuel	ATL	2012	110	175	29.0	0.63	2.17
28	Patrick Robinson	NOR	2012	99	190	24.2	0.52	2.15
29	Tim Jennings	CHI	2013	111	182	28.5	0.61	2.14
30	William Moore	ATL	2010	117	178	31.1	0.66	2.11
31	Richard Sherman	SEA	2013	125	188	31.6	0.66	2.10
32	Malcolm Jenkins	NOR	2010	105	174	29.2	0.60	2.07
33	Devin McCourty	NWE	2010	110	166	32.1	0.66	2.06
34	Brandon Flowers	KAN	2011	95	189	24.6	0.50	2.04
35	Eric Berry	KAN	2013	134	197	33.4	0.68	2.04
	Ed Reed	BAL	2004	358	192	34.2	1.86	5.45
	Darren Sharper	NOR	2009	376	185	31.4	2.03	6.47
	Deion Sanders	SFO	1994	303	198	-	1.53	-
	Charlie McNiel	SDG	1961	349	177	-	1.97	-
	Night Train Lane	LAR	1952	298	171	-	1.74	-