Manning best ever in 'TDs added'

— -- Peyton Manning has made the history books again, and his latest achievement is more impressive than you think. While Brett Favre needed 10,164 passes to reach 508 touchdowns, Manning's NFL-record 509th TD pass happened in just 8,659 attempts (his 510th TD occurred on his 8,669th pass). But that's not all.

To get a more complete appreciation for Manning's accomplishment, we need a better way to compare and contrast his stats with those from other eras. It's not easy to compare NFL football across the decades. The changing rules, offensive styles and defensive schemes can really define the opportunities for quarterbacks and other players.

Can we honestly compare Manning's touchdown totals to those of Sonny Jurgenson or Bob Griese, who were throwing touchdowns before offensive linemen were allowed to hold and defensive backs were prohibited from touching a receiver after five yards?

We can try, with a new stat: touchdowns added. And this is where Manning truly shines.

(Feel free to skip the fine print in these two paragraphs, if you like, and cut to the findings below. To calculate touchdowns added and to make the numbers a little more comparable -- more apples to apples -- 2013 was set as the base year, the season all other seasons would be compared to. The rate of touchdowns to pass attempts was calculated for every season from 1932 through to the present. Each quarterback's touchdown rate was then adjusted to what it would have been in 2013. The adjusted TD rate and the actual pass attempts for each QB were used to determine his new adjusted TD rate -- and that number was compared to what an average QB in 2013 would have done with the same number of pass attempts. The difference between a QB's adjusted total and the average or expected total produces the QB's "touchdowns added" for that season. Once touchdowns added are calculated for each of a QB's season, then his total career touchdowns added are added up.

For example, Dan Marino threw 48 touchdowns in 1984 -- a touchdown rate of 8.5 percent. That is the same as a touchdown rate of 8.9 percent in 2013. An average QB in 2013, throwing the same number of passes as Marino, would have thrown 29 touchdowns, so Marino threw the equivalent of 50 touchdowns. Marino threw 21 more touchdowns than an average QB in 2013 would have -- the fourth-best season in history.)

The leader in the clubhouse for career touchdowns added is Manning, and it is not close: He has 157 touchdowns added. That means Manning has thrown 157 more touchdowns than an average QB would have in the same number of attempts. Favre, even with 1,500 more attempts, has 112 touchdowns added -- and he is second on the list. In fact, while chasing Favre's total TD mark, Manning actually surpassed Favre on the career touchdowns added list at the end of the 2010-2011 season, when he had 113 touchdowns added. So even adjusting for the current era of inflated TD totals, Manning is still the most prolific TD passer in history.

Manning also owns two of the three highest seasons in terms of touchdowns added. Tom Brady owns the top mark with 27 touchdowns added in 2007, but Manning is right behind him with 26 in both 2004 and 2013 -- and he is the only QB to have two of the top 10 touchdowns added seasons.

The top five in touchdowns added is the same as the top five in career touchdowns, but in a fairly different order. Brady leaps both Brees and Marino in touchdowns added, and Brees is only six touchdowns added behind Marino, so he may catch him within the next season. Some other QBs who accumulated a lot of touchdowns added include Joe Montana (12th all-time with 51), Sonny Jurgenson (18th all-time with 41), and Manning's hero, Johnny Unitas (35th all-time with 23).

Of the active QBs, Manning, Brady and Brees lead the way, with Rodgers quickly catching up and Philip Rivers a distant 5th with 100 fewer touchdowns added than Manning.

Rodgers appears to be the only active QB with a legitimate shot at catching Manning, but even he lags Manning's pace -- at the same point in his career as Rodgers is now, Manning had already had 82 touchdowns added.

Some QBs, however, are on the wrong side of the comparison and have negative totals, meaning they threw fewer touchdowns than could be expected of them. After all the adjustments, the QB who under-performed the most, given the opportunities he was given, was Kerry Collins, who, in 17 seasons and 6,261 pass attempts, threw 49 fewer touchdowns than an average QB would have. He is joined at the bottom of the list by Jack Kemp with 38 fewer touchdowns and Rick Mirer with 33 fewer.

The touchdowns added metric gives Manning's current totals and pursuit of Favre the proper context. This number tells us that no other QB has been as proficient for such a sustained period of time as Manning has been, and he will likely remain atop that list for some time to come.