Explaining Strength Index and OSI

By Jon Fass
Posted:  Tuesday, September 24, 2019  10:55 am

Every offseason, it seems, the road which leads to the high school football playoffs is under repair.  The idea behind so many changes is to make sure that the teams that are most deserving of a playoff berth are rewarded with one.

This season's new wrinkle is Opponent's Strength Index, or OSI.  I will do my best to explain that here, but for an official breakdown of how everything is calculated, be sure to read the NJSIAA's 2019 Football Regulations.

It is quite confusing, to say the least, so if you're having a hard time wrapping your head around all of this, you should know that you're not alone.

The two new components are Strength Index and Opponent's Strength Index.  One is dependent on the other, but they are two distinctly different values, and having the word Index in both is not a good idea.  Until these names are officially changed, I will refer to them as Strength Index and OSI.

Let's begin with Strength Index.

Each team opens the season with a starting Strength Index.  After each game is played, the Strength Index will increase or decrease based upon the score.  I'll use Hawthorne as an example.

The Bears' Strength Index was 58.87.  After their first game against Lyndhurst, the Strength Index dropped to 57.56.  After the second game against Elmwood Park, it went up to 59.74.  And after their third game against Harrison, it went up to 63.

But -- and here's the confusing part -- that doesn't matter when calculating Hawthorne's OSI.  OSI is based on the Strength Index of a team's opponents.  In a win, a team gets 100% of the opponent's Strength Index.  In a loss, a team gets 50%
of the opponent's Strength Index.  And in a tie, a team gets 75% of the opponent's Strength Index.  Those numbers get averaged, and that's how you calculate a team's OSI.

Now back to Hawthorne.  They lost to Lyndhurst, but defeated both Elmwood Park and Harrison.  After the games that were played this past weekend, Lyndhurst's Strength Index is 65.67, Elmwood Park's Strength Index is 24.27, and Harrison's Strength Index is 45.76.

So for Hawthorne, the loss to Lyndhurst is worth 50% of their Strength Index of 65.67, which is 32.84.  The wins over Elmwood Park and Harrison are worth 24.27 and 45.76, respectively.  The average of those three values is 34.29, and that's how Hawthorne's OSI is calculated.

Does that make sense?

And keep in mind, w
ith every game played, a team's Strength Index changes, so the value Hawthorne gets for beating Harrison is 45.76 today.  But as Harrison's Strength Index changes over the course of the season, so does the value that Hawthorne receives for that win.

I will continue to provide more clarification as the season goes on, but of course, if you have questions, please feel free to reach out.

Jon Fass can be reached by emailing jon@gridironnewjersey.com.