Not all passes are created equally. 

A two-yard completion on 3rd-and-14 means less in a game than that same two-yard completion does on 3rd-and-1. That type of idea is what the basis of our Net Expected Points (NEP) metric is built from. 

If you're unfamiliar, NEP measures the value of each play on the field based on how an average team would be expected to score in each scenario using historical data. On top of NEP, we track Success Rate during the season, which is the percentage of plays a given player is involved in that positively impacts his NEP.

Success Rate is an interesting stat to look at across all the offensive skill positions, but perhaps the most so at quarterback. Typically, a wide portion of the football viewing public would take a quarterback’s Completion Percentage as a de facto “Success Rate” -- did he complete the pass or not? But, as we know, with the two different two-yard completions on third down, that view doesn’t exactly hold true.

For the most part -- even from those who look at such things -- Success Rate and Completion Percentage are viewed as different unrelated statistics. But there’s a part of each stat that works together, in a way, and that intersection is what we’re going to explore here.

Because all completions are not created equally, not every completion is going to be labeled a success. What that leaves is a difference between Completion Percentage and Success Rate that could help show how many empty completions some quarterbacks have made to pad their stats. Also, on the other side, it can show which quarterbacks make the most of their completed passes by having a higher number of those completions labeled successes.

By slightly adjusting numberFire’s Success Rate -- the statistic, as recorded, measures successes per drop back, so I adjusted to measure for total successes per attempt to match up with the same baseline as Completion Percentage -- we can see which quarterbacks got the most and least out of their completions.

Successful Completion Percentage

There were 37 quarterbacks  measured in this study, all of whom attempted at least 200 passes during the 2015 season. Among those quarterbacks, the average Successful Completion Percentage -- or, successful completions divided by the number of attempts made -- was 49.5 percent, well below the 62.5 percent average for Completion Percentage. Only 18 of the 37 quarterbacks were actually successful on more than 50 percent of their attempts in 2015, and only five were above 55 percent. 

Below are the top-10 quarterbacks from 2015 in Successful Completion Percentage:

Russell Wilson had the third highest Completion Percentage during the regular season, but jumps up to the top of the list when looking at successful throws versus total attempts. Wilson is one of three quarterbacks who come in over 55 percent, with a 0.6 percent lead over Ben Roethlisberger. The top five players in Successful Completion Percentage all jumped up in ranking when compared to raw Completion Percentage, showing both the ability of these quarterbacks and the inflation at the top of the Completion Percentage rankings.

Meanwhile, Kirk Cousins dropped down to seventh in Successful Completion Percentage, despite completing the highest percentage of passes during the regular. In other words, a good number of his completions weren't actually successful.

Biggest Gainers

When comparing Completion Percentage and Successful Completion Percentage player rankings, both Cam Newton and Jameis Winston tied with the highest jump, moving up 15 spots in ranking. A total of eight quarterbacks moved up at least 10 spots from Completion Percentage rankings to Successful Completion Percentage rankings but, of course, not all of those differences are equal -- Winston went from 32nd in Completion Percentage to 17th in Successful Completion Percentage, while Newton went from 28th to 13th. 

The most impressive jump might have come from Brock Osweiler, though. The part-time Broncos starter was just 22nd in Completion Percentage, but ranked 9th in Successful Completion Percentage. Osweiler also rated well by the amount of his completions that were successes -- 84.1 percent, which was eighth best among quarterbacks.

Carson Palmer made a similar jump towards the top, going from 15th in Completion Percentage to 4th in Successful Completion Percentage. Palmer also led all quarterbacks in successes per completion at 87.1 percent.

Biggest Fallers

Four quarterbacks saw a drop of 10 spots or more from Completion Percentage to Successful Completion Percentage. Teddy Bridgewater had the biggest drop off among quarterbacks, a difference of 18 spots. Despite Bridgewater completing 65.3 percent of his passes during the season, only 46.5 percent of those attempts were deemed successful. We took a look at Bridgewater and the odd system offensive Norv Turner was running during the season for a reason why the second-year player was struggling to meet expectations. In that system, he was encouraged to find the check-down, and while that led to the 10th highest Completion Percentage in the league, many of those completions meant very little on the field.

Blaine Gabbert was also around league average with his Completion Percentage, ranking 18th, but he was 30th among these quarterbacks in Successful Completion Percentage during the season. It’s possible Gabbert competes for the starting job in San Francisco this season -- or becomes the starter should Colin Kaepernick's trade request be granted -- but these numbers reflect poorly on what looked like a surprising improvement by raw numbers. 

Gabbert actually happened to be one of the most efficient quarterbacks in the red zone this season, but to rate this poorly on the full season gives a glimpse of how he performed across the other 80 yards of the field. Take into account that red zone efficiency is highly variant from year to year, and the prospect of Blaine Gabbert, NFL Starter continues to be unappealing.

Failed Completions

Up to this point, we’ve measured the amount of successes per attempt for these quarterbacks. As noted above, the numbers for Successful Completion Percentage come in well below those for raw Completion Percentage, which means there are completions each quarterback makes that add no or negative value by NEP. 

We’re going to call those "Failed Completions", and look at some of these quarterbacks’ Failed Completion Rate. We’ll be keeping the denominator in the equation as “attempts”, so what’s below is the percentage of overall passes from a quarterback in 2015 that resulted in a Failed Completion. The average among this group was 13 percent.

Seven quarterbacks had a Failed Completion on less than 10 percent of their attempts. All of these players are the ones who saw the biggest jump in Successful Completion Percentage ranking, such as Winston, Newton, Palmer and Osweiler. The only quarterback to be listed among the top-10 gainers for Successful Completion Percentage and not have a Failed Completion Rate under 10 percent was Marcus Mariota, who was ninth among quarterbacks at 10.3 percent.

Nine quarterbacks had a Failed Completion Rate over 15 percent, led by Bridgewater at 18.8 percent. The next worst quarterback was Gabbert at 17.4 percent, followed closely behind by Nick Foles at 17.2 percent. Foles has gotten mostly passed over in this analysis because he was neither a big gainer nor loser. Instead, he was at the bottom of the rankings for both of these statistics -- 35th in Failed Completion Rate and last in Successful Completion Percentage. He was the only quarterback to fall under 40 percent. 

It should also be noted here that the top two quarterbacks in completion percentage, Cousins and Drew Brees, were among the nine quarterbacks with a Failed Completion Rate over 15 percent. This makes sense intuitively -- their high Completion Rates were bolstered by easy completions. 

If you'd like to see the entire breakdown, below is the full list of quarterbacks, showing their Successful Completion Percentage, Failed Completion Rate and Completion Percentage, along with the ranking of each: