The S&P statistic, developed by Bill Connelly, is made by compiling two different numbers: A team's success rate and Equivalent Points Per Play (PPP).
Let's break each of these down individually.
How much is eight yards worth to an offense? Obviously, the answer changes depending on the situation. If it's first-and-10, eight yards is valuable. If it's third-and-26, eight yards doesn't do so much. This is the reason that looking at a team's total yardage oftentimes is misleading, as it doesn't tell us if teams picked up valuable yards or simply junk yards.
This is where Connelly's success rate comes in. He considers each play a "success" for the offense if it meets the following criteria: 50 percent of needed yards on first down, 70 percent of needed yards on second down, or 100 percent of needed yards on third or fourth down.
So on first-and-10, a "success" is if the offense gains five yards or more. On second-and-20, a "success" would be 14 yards or more. On third-and-26, only 26 yards or more would be considered a "success."
The average success rate for Division-I college teams through week two of the 2009 college football season was 41.1 percent.
Why is success rate important? In short, it measures a team's efficiency.
As Connelly says, "If a team racks up a whole bunch of yards but only has a success rate of, say, 35 percent, they probably are going to struggle to win the game unless their defense is great. Racking up a ton of yards in 3-4 plays while forcing yourself into 3rd-and-8s the rest of the game is not a good recipe for scoring a lot of points."
The "success" rate number, in S&P ratings, is always listed as a decimal. So a 41.1 percent success rate would be .411. A 100-percent success rate (the highest a team could achieve) would be 1.000.
OK, so that takes care of how efficient a team is during a game. But what about a team's explosiveness, or ability to make big plays?
That's where the PPP rating comes in.
Yes, yards per play would be one way to measure explosiveness, but it wouldn't tell us the whole story. After all, as Connelly says, "... all yards are not created equal. A 10-yard gain from your 15-yard line to your 25 is not the same as one from your opponents’ 10-yard line to their end zone, or one from your opponent’s 40 to their 30, advancing into field goal position."
This is where his equivalency points system comes into play. After breaking down the numbers, Connelly has come up with a point value for each yard line that represents the number of points an average college football team scores from that yard line.
To get a point-per-play number, Connelly simply subtracts the old yard line from the new one.
Let's do an example. Let's say KU gains 10 yards from its own 20-yard line to the 30. Let's say the equivalent point value of a team's own 20 is 1.20 and the equivalent point value of a team's own 30 is 1.45 (I'm making these numbers up). KU's PPP on that play would be 1.45-1.20, or .25.
After Week 2 of the 2009 season, the average PPP of college football teams was 0.34.
For a team's S&P ranking, Connelly simply adds the success rate number (S) to the PPP number (P). Using this, we can judge teams offensively based on both their efficiency and explosiveness.
The average S&P for a team after Week 2 of the 2009 college football season was 0.749 (excluding all FBS vs. FCS games).
Connelly's S&P+ rankings are weighted, where 100 is average for an NCAA team.