How Does 4th Down Aggression Affect Success Rate?

And what does that question even mean? Picture this, if you will.

Baylor* faces 3rd down and 8, somewhere near midfield. It’s an obvious passing situation, and almost any defense will be thinking pass first. Instead, offensive coordinator Jeff Grimes calls a wide zone run, one of Baylor’s staple plays in the 2021 season. Abram Smith follows his blocks, finds a hole, and gains 6 yards. 4th and 2. What now?

*I’m using Baylor as an example because at this stage of my writing career I assume the vast majority of my readership are Baylor fans. If you’re not a Baylor fan, this scenario was not an rare occurrence.

Well, if my hypothetical played out the way it usually did for Baylor in 2021, they went for it on 4th down. Baylor attempted a 4th down conversion on 35 out of 106 opportunities. That’s 33.02%, 9th highest in the nation. (Only Army, Ole Miss, Air Force, South Alabama, Memphis, Charlotte, Tulane, and UCLA attempted 4th down conversions at a higher rate.) Baylor was pretty good on 4th down too, converting 22 of those 35 attempts. So this brings up a key question: Was Baylor’s 3rd down play a successful play?

Success Rate

Bill Connelly’s main metric, SP+, bases its data on his Five Factors of football success: efficiency, explosiveness, field position, finishing drives, and turnovers. The first factor, efficiency, measures how well an offense stays on schedule and avoids negative plays, and Success Rate measure an offense’s ability to do that. Success Rate is the percentage of plays that are successful, and whether a play is successful depends on the down, distance, and yards gained:

• On 1st down, a play is successful if it gains at least 50% of the required yardage to get another 1st down.
• On 2nd down, that percentage jumps to 70%.
• On 3rd and 4th down, you need 100% of the required yardage to be credited as a successful play.

So on 1st and 10, a play gaining 7 yards would be successful. On 2nd and 5, a 2 yard gain would not be successful. And in our original scenario, a 6 yard gain on 3rd and 8 would also be considered unsuccessful.

But should it have been? Last season, Baylor’s Success Rate in power situations (3rd/4th down and 2 or less) was 74.29%, meaning that 4th and 2 wasn’t exactly a scary proposition. Baylor was comfortable calling a run on 3rd and 8 in the first place because, even if they didn’t pick up the 1st, they expected to gain enough yards to make 4th down manageable. From a lay perspective, one could call the play successful. But was it?

The Experiment

To answer this question, I needed some data. First, I got each team’s Success Rate and total efficiency from CollegeFootballData.com. (For efficiency, I used CFD’s PPA stat, a stat that uses historical data to assign expected point values to individual plays.) I also got each team’s 3rd and 4th down stats to determine how often a team attempted a 4th down conversion.

I measured each team’s 4th Down Aggression by taking the number of times an offense failed on 3rd down. That was my denominator, as it was the total number of chances a team had to go for it on 4th down. Then I took the number of 4th down conversions attempted and made that my numerator. From there, I got a percentage that I could use as that team’s 4th Down Aggression.

With my remaining data, I created two scatterplots.

In this graph, each dot represents a team. The line running through the data is called the best-fit line, which is the computer’s best guess as to the relationship between the independent variable (Success Rate) and the dependent variable (PPA/play). Finally, R² is a measure of how well the data adheres to the best-fit line; here, pretty well!

The second chart had pretty similar results to the first. For this study, my interest is in the teams well above the best-fit line. Those teams finished the season with PPA significantly greater than one would expect from their Success Rate. So could 4th down aggression be the reason for that discrepancy?

To answer this, I took the difference between each team’s actual PPA or PPA/play and their expected number based on the best-fit line. Then, I compared that Error to each team’s 4th Down Aggression number that I calculated earlier. Would there be a correlation between 4th Down Aggression and overperforming your Success Rate?

Results

That is…about as uncorrelated as you can get. Remember R², the numeric metric for correlation? Here, it’s 0.029, meaning 4th Down Aggression explains only 2.9% of a team’s overperformance of PPA.

What Did We Learn?

Maybe the takeaway is something we knew all along: 4th downs are risky. And that’s why 4th down has a different set of rules; no one punts or kicks field goals on 1st-3rd downs (end of half situations notwithstanding). As good of a short yardage team Baylor was last year, one of out of every four power situations resulted in failure. And one in three of Baylor’s 4th down attempts ended the same way.

So to answer our earlier question, while gaining 6 yards on 3rd and 8 is far from the worst case scenario, and the play had 1st down upside because the defense was playing pass first, it simply isn’t a good idea to leave things to 4th down, even 4th and 2.

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