Completion percentage is a tricky stat in the NFL. It can be a decent measure of quarterback performance, as long as it’s used in the right context.

That stat, like any single statistic, has its flaws, especially in this era of passing. Passes in this era are getting schemed shorter and shorter, which allows completions to happen at a higher rate and has boosted completion percentages over the past few years. Using the NFL’s baseline of 224 attempts in a season, six of the NFL’s 10 highest completion percentages in a season have comes in the past five years. A seventh came from Drew Brees in 2009. This list includes Sam Bradford’s record-setting 71.6 percent completion percentage in 2016.

The biggest flaw in completion percentage is that it treats all passes equally. But if you’re reading this site, there’s a good chance you know that isn’t the case. That’s one of the reasons we use our Net Expected Points (NEP) metric. The basic idea behind NEP is a five-yard pass means more on 3rd and 2 than it does on 3rd and 14, and each play should be credited as such.

While we measure each play by NEP, we also measure Successes -- whether or not a play positively impacted NEP. That allows us to get a Success Rate for all players, but in this case we’ll focus on quarterbacks.

Normally when we measure and use Success Rate here, we do it per drop back. That is quite useful on its own, but it doesn’t exactly give us the same type of measurement as completion percentage. So starting last year, I took each quarterback’s Successes and measure them against his passing attempts to get a better comparison to his completion percentage. That gives us what we’ve called Successful Completion Percentage and for the study below, we’ll look at how 39 quarterbacks fared this season.

Successful Completion Percentage

Successful Completion Percentage is pretty simple and can be calculated by taking a quarterback's successful completions and dividing them by the number of attempts made.

When measuring the amount of successes instead of completions per attempt, it changes the leaderboard a bit. It should be of little surprise that Bradford’s 71.6 percent completion percentage doesn’t exactly hold up when judged by success, though Bradford does narrowly stay in the top 10 overall. There should also be no surprise that a certain Atlanta quarterback finished first in 2016. Below is the top 10 from this past season.

In 2015, Russell Wilson led all quarterbacks with a 56.7 percent Successful Completion Percentage, 1.7 percent below what Matt Ryan put up this season. Another stat Matt Ryan leads this year? No way.

Ryan, in an overlooked season last year, was third in Successful Completion Percentage and is just one of four quarterbacks to be in the top 10 in both 2015 and 2016. The other three are Kirk Cousins (seventh, fourth), Russell Wilson (first, sixth), and Andy Dalton (fifth, ninth).

Biggest Gainers

The three biggest gainers all went from mediocre or bad in completion percentage to the top 10 in Successful Completion Percentage. Matt Barkley and Jameis Winston were both in the mid-to-low 20’s in raw completion percentage, but they were among the best at getting the most out of their completions. This is the second year in a row that Winston is one of the biggest risers -- though went from bad to average last season and from average to good in 2016.

Of the three quarterbacks who jumped eight spots, only one was particularly good, though most would classify Carson Palmer’s 2016 as a disappointment after an MVP-worthy 2015 campaign. This is also the second year in a row that Palmer has been one the top risers, mostly because of the vertical aspect of the offense in Arizona. There’s a limited amount of checkdowns in that offense and when there are, they usually go to David Johnson, who is quite good as a receiver.

Biggest Fallers

Our biggest fallers include three players in the top 13 of raw completion percentage and two Los Angeles Rams quarterbacks. Joe Flacco led the league in pass attempts, which greatly increases the probability of completions that don’t accomplish much.

During his first year in an Adam Gase-run offense, Ryan Tannehill was able to take advantage of some easier throws and more open throws. While that did help some in production -- Tannehill increased his yards per attempt from 7.2 in 2015 to 7.7 in 2016 -- it also led to some passes that didn’t add much to the offense.

Those types of passes have been the speciality of Alex Smith. Smith, notoriously known for his willingness to checkdown, has mastered the art of the completed pass for completed pass’s sake. Smith completed over 67 percent of his passes in 2016, but just over half of his pass attempts resulted in a Success.

Then there’s those Rams. Los Angeles had the worst passing offense in the league, per Adjusted NEP per play, by a wide margin -- the 31st ranked Houston Texans were closer to the 20th ranked Buffalo Bills than the last-ranked Rams -- so it’s not a stretch to imagine there was little of value on passes from the quarterbacks.

Failed Completions

Another thing we can look at here is the difference between raw and Successful Completion Percentage, which gives us the percent of Failed Completions a quarterback had during the season.

The average quarterback this season had a Failed Completion on 13.4 percent of attempts. Of course, there were a few who far exceeded that average. Here’s the five highest Failed Completion rates of the season:

Jared Goff was easily the worst quarterback when judged by successes and failures. Goff was last in Successful Completion Percentage and was also the only quarterback to have Failed Completions on over 20 percent of his attempts.

Meanwhile, the all-time leader in completion percentage had the second-highest rate of Failed Completions this season, which helps show how flimsy completion percentage can be as a measure of ability. The top (bottom?) five is then rounded out by two of the biggest fallers from the section above along with Robert Griffin III, who struggled in his limited time starting for the Cleveland Browns.

Below is the full list of quarterbacks and their rankings for Successful Completion Percentage, Failed Completions, and regular completion percentage: