How do you measure a truly great starting pitching season? Having an ERA that starts with a “1” generally qualifies, but there’s some obvious issues with that. First, that’s happened 82 times in the last 100 seasons, making it notable but perhaps not unthinkable. Second, obviously, are the flaws inherent to ERA, most importantly that it’s not adjusted for ballpark or league. Pedro Martinez (2000), Sandy Koufax (1964) and Carl Mays (1917) all had an ERA of 1.74. Clearly, none of them were facing the same kind of offenses.

You could, if you wanted, go by WAR. Steve Carlton‘s 1972 and Martinez’ 1999 make sense atop the list, but convincing people that Bert Blyleven‘s 1973 was the third-best season ever or that Bob Gibson’s legendary 1968 was merely his third-best season seem like tougher sells. Besides, since that’s a counting stat rather than a rate stat, it means no modern-day pitcher will ever be able to come close, because it seems pretty safe to say that we aren’t seeing a starting pitcher top 320 innings again, as both Carlton and Blyleven did.

FIP? That’s better, though still imperfect. Martinez, again, and 1984 Dwight Gooden top the leaderboards there, followed by a pair of guys essentially playing a different sport, Walter Johnson and Pete Alexander in 1915. (In 1915-16, Johnson pitched 706.1 innings; he allowed one home run. It was, as were 24 of the 97 total dingers he allowed, an inside-the-park job.) FIP also assumes some league-average inputs, and if we want “best-ever” perhaps we don’t want to assume any kind of average; like WAR, you’d also have a tough time winning a bar argument with something that you need to explain formulas for.

Enough setup, then. To the point, now. Clearly, there’s many different ways to do this, and no obvious, unassailable answer. You could make an argument for probably a dozen different years as the “best” starting pitching season of all time. What I’m doing, today, is to break it down into the most important things a pitcher can do that are more or less entirely within his control:

Get strikeouts

Limit walks

Limit homers

Get ground balls

If you’re good at any of those things, you might have a big league career. Be good at two, and you might have a productive big league career. Be good at all of them, to an exceptional degree, and you’re a superstar.

Let’s go back to the qualified pitcher seasons since 1915, of which there are 7,464, and set some requirements for those four items. There’s 149 seasons (0.019%) where a starter has struck out at least 25% of the hitters he faced. Of course, the sport is striking out so much more often now than it used to, making raw whiff numbers a bit skewed towards the present-day pitchers, something of the inverse of how WAR is going to work better for the older guys who pitched more often. (To use a favored example, Gibson’s career 19.4 K% is identical to luminaries Chris Capuano, Shaun Marcum and Mike Trombley.) So I asked our pal Jeff Zimmerman to help me figure out how many times a K% was at least eight percent higher than the league average for that season. The answer, as it turns out, is 202 times. It’s slightly more often than using straight K%, but less often than I think I expected.

669 times, or 0.089%, a pitcher has walked fewer than five percent of the hitters facing them. Only 18 times — 0.002% — has a pitcher topped 25% in strikeouts and kept walks below five percent, and there’s some great seasons in there. Martinez’ 1999 and 2000, of course. Matt Harvey, last year. Curt Schilling shows up three times. Cliff Lee twice. Only one of the 18 is from before 1999, which is Koufax’ 1963. We can do better than that. We don’t just want good, or very good. We want truly elite, and that means a fantastic ability to prevent home runs. Four of those seasons had more than one home run per nine innings. Ten of them were above 0.50/9. Still very good, and yet now out of the mix.

We’re left with four seasons. Now we’re going to split that in half. An out is an out, and so a a grounder may not be specifically more valuable than a fly out or a pop out. But it is exceptionally difficult to hit a ground ball over the fence, and so ground ball pitchers are often seen as more desirable than fly ball types; you can pitch a ground ball pitcher anywhere, but the fly guys look a lot different based on what park they’re in. Let’s limit our pitchers to only seasons where the ground ball rate has been at least 50%; this, unfortunately, goes back only to 2002, when batted ball data was first available. 218 pitchers have done that.

Now, let’s combine it all, which leaves us with K rate above 25%, BB rate below 5%, and ground ball rate above 50%. We’re left with two guys. They’re both doing it right now. You probably already know who they are:

Name K/9 BB/9 K/BB HR/9 K% BB% ERA- FIP- xFIP- ERA FIP xFIP GB% Clayton Kershaw 10.76 1.25 8.58 0.40 31.80% 3.70% 50 49 54 1.78 1.75 2.01 55.00% Felix Hernandez 9.68 1.60 6.06 0.35 28.30% 4.70% 51 55 62 1.95 2.07 2.38 56.10%

It’s probably not earth-shattering to point out that Clayton Kershaw and Felix Hernandez are the best pitchers in baseball. They’re all but guaranteed to win the Cy Young awards assuming health; Kershaw could have been headed towards a streak of four in a row if not for the infatuation with R.A. Dickey’s knuckler and story in 2012. Neither one is even 29 yet, and we’re talking about them both as though they’re obvious Hall of Famers someday.

What this is, really, is just showing another way that they’re the best pitchers in baseball. They’re two of a very, very small group — 0.002% in the last 100 seasons — with the ability to get whiffs at the pace they’ve done while keeping walks to a minimum. They’re two of only four to limit homers below 0.5/9 in that group, which is not unrelated to the fact that they keep the ball on the ground so well. Due to the limitations of batted ball data, we can’t say with complete certainty that no one has done this before, but we can make an educated guess. We can say it’s insanely, unbelievably good.

If you prefer this in a visual format, here’s a graph of every qualified starting pitcher season since 2002, all 1,136 of them. Obviously, a huge majority of pitchers are clustered in the middle, getting grounders between 35 and 50 percent of the time, and with K%-BB% between 5 and 15%. If you’re at the bottom left, you’re probably in trouble. (No, that’s not a mistake — there are four guys who managed to qualify with negative K%-BB% marks.) If you’re one of the two dots in the extreme upper right, well… you’re doing it right:

The point, again, is not simply to say, “hey, Kershaw and Hernandez are good.” You know that. What might somehow be not fully appreciated is just how good, and at how many different things. Perhaps, because offense is so low right now, it’s more difficult to stand out as a pitcher. Or perhaps offense is so low because of how great pitchers like Kershaw, Hernandez, Chris Sale, Stephen Strasburg, Yu Darvish, etc. all are. Or maybe, and possibly most likely, we’re seeing two of the best to ever pitch, , we’re seeing them at their best, and even within the context of the great things they’ve done before, they’re outdoing themselves. They’re doing things we’ve just about never seen before, ever. We should be thankful every single time they take the mound. This isn’t just “best pitcher in baseball” performance; this is “best you might see in your entire lifetime” kind of stuff.