What are the Most Predictable Year-to-Year Stats for Quarterbacks?
When projecting player stats, it can be tempting to simply look at the previous season as a sign of things to come and build expectations accordingly. While this can be a valuable exercise, if the analysis is predicated on the wrong stats, the effort will be all for naught.
This study will shed some light on which commonly referenced stats are the most reliable indicators of future quarterback performance, as well as which commonly referenced stats might be misleading.
Because so many things can change in the NFL from year to year, only quarterbacks from 2010 on that remained on the same team in consecutive years and attempted at least 200 passes in both years will be considered. In a study that can be inherently sensitive to outliers, eliminating something as drastic as a team change allows us to limit some of the noise. Although 200 pass attempts is an arbitrary cutoff, an average of at least 25 attempts per game over half of a season should be sufficient enough to put a player's performance under scrutiny.
Using this methodology, we end up with 113 instances that meet the volume threshold. This sample will be used to determine which of the commonly referenced stats are the most useful when trying to project a quarterback's upcoming season. We can do this by comparing how well each commonly referenced stat correlates from one season to the next.
Why One Type of Stat Has a Stronger Correlation Than the Others
The table below lists the year-to-year correlations for 19 separate quarterback stats using the methodology described above.
|Stat||Year to Year Correlation|
|Total Passing Yards||.46|
|Total Passing TD||.43|
|Total Fantasy Points||.42|
|Total Pass Attempts||.40|
Considering that they simply represent a player's average performance instead of his season-long totals, it’s no surprise that per-game averages trump season-long totals in terms of predictability. If a quarterback misses a handful of games in one of the seasons, his season-long totals are obviously going to quickly plummet. Given the weak correlation of games played from one year to the next, we know that’s a real possibility.
With an understanding of why per-game stats correlate so strongly from year to year, we can take a closer look at which are the most reliable in predicting the upcoming season for a given quarterback.
Of the per-game volume stats, passing yards per game is the only one that is correlated strongly from year to year. All of the other per-game volume numbers (pass attempts per game, fantasy points per game, and touchdowns per game) have a moderate year-to-year correlation, but it's not to the point where you'd look at the previous season's numbers and feel comfortable projecting those numbers forward.
Also note that touchdowns carry so much weight when it comes to fantasy scoring that their relatively low year-to-year correlation brings down the correlation for fantasy scoring as well.
The Smart Way to Use Efficiency
The relatively weak year-to-year correlations of the efficiency metrics lends some insight into why yardage is so predictable even though attempts might fluctuate from one year to the next. In fact, other than completion percentage, we cannot rely on any of the commonly referenced efficiency metrics, such as yards per attempt, adjusted yards per attempt, touchdown rate, and interception rate, as predictable stats based on the previous season's data.
Consider the yearly touchdown rates of Aaron Rodgers, who ranks first among active quarterbacks in career touchdown rate (6.4%) and is widely regarded as one of the most efficient quarterbacks in the league:
Even the most efficient quarterbacks can have some huge swings in their touchdown-scoring tendencies from year to year. As such, the best application of efficiency metrics is probably to use the past season's data to gauge whether or not a quarterback is due for regression (be it positive or negative) to the league average, or to his individual average if there's a large enough sample.
Beware of Rushing Numbers
The fact is that most quarterbacks don’t run very much, so the majority of them are always going to have very low rushing totals from one year to the next -- this explains the very strong correlation of rushing yards from year to year. If we narrow the sample down to quarterbacks who rushed for 200 or more yards in a season, the correlation drops to .70. Quarterbacks that rushed for 300 or more yards in a season saw a correlation of just .56, while the 16 quarterbacks that rushed for 400 or more yards in a single season since 2010 saw a statistically insignificant year-to-year correlation of .29.
We see the same effect when looking at rushing touchdowns. If a quarterback scored at least four in a given season, the rushing touchdown correlation fell to just .39.
The Bottom Line
Whether you're projecting player stats for an entire season or a single week, there are countless factors to consider outside of what happened in the past. During the offseason, though, numbers from the previous year are often the only hard data that we have to point to. Those numbers can be used as a reliable point of reference when the right factors are considered, and we now have an understanding of which stats are a sign of things to come for quarterbacks, as well as which ones might not serve as a future indicator.
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