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So far this offseason, we've taken a look at pitchers who attacked the strike zone the most, who improved their Stuff+ the most, and who saw the biggest increases in K-BB%, amongst others. This article will take yet another slice of the data to see what there is to see.
In this case, we'll take a look at SIERA -- and to a lesser extent FIP/xFIP -- and compare it to ERA in order to see if there are pitchers who may have over or underperformed relative to their ERA.
For those that aren't familiar, SIERA stands for Skill-Interactive ERA and it aims to better express in numerical terms how good a pitcher was.
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Overview of SIERA for Starting Pitchers
The SIERA calculation incorporates balls in play and is a slightly better predictive metric than FIP or xFIP (although it is much more a look-back than a look-forward calculation). The point is to find pitchers that may be regression candidates in the coming season and help managers make better decisions when it comes time to decide between two or more players.
A couple of things to note are that some pitchers can tend to over or underperform consistently over time, so they may not be regression candidates (most of the pitchers discussed below do not have the track record to determine this yet, though). We looked at starting pitchers with 50+ IP in 2023 for this analysis and discussed the more interesting ones. Below is a table from FanGraphs that outlines how good or bad a pitcher's SIERA was.
SIERA vs. ERA Analysis: Overachievers
Note that the difference between ERA and SIERA is indicated in parenthesis to the right of each pitcher's name below. Also, Clayton Kershaw and Justin Verlander are italicized in the above table because they are very fantasy-relevant names, but they're not discussed below. So, first up...
Blake Snell, 2.25 ERA / 4.06 SIERA (-1.81), 66 ADP
If you've been prepping for your fantasy baseball draft for the past several weeks, you've probably read up on Blake Snell and know a few things about him by now. You probably know that, despite winning the NL Cy Young last season, he's got terrible command (13.3% BB%, 4th percentile; 13th percentile Location+).
You may have also read that, other than his two Cy Young seasons, he hasn't pitched more than 130 innings in a single season. Well, this article will point out another reason to be cautious on the southpaw.
Last season, Snell's 2.25 ERA went a long way to securing the Cy Young Award, and his K% was excellent too (31.5%, 94th percentile). Taking a look at his SIERA of 4.06, however, it seems maybe his ERA should not have been as sterling as it was. SIERA takes into account park factors, so pitching his home games in Petco Park surely helped Snell -- to an extent. In 2021 and 2022, his SIERA was lower than his actual ERA despite pitching in San Diego.
What jumps out to me is that his left-on-base % (LOB%) was fourth-highest in the MLB at 86.7%, well above his career rate of 78.2% and league average of 71.9%. He didn't get that many ground ball double-plays (13) when compared to league leaders (20+), so it appears his ability to strike batters out is what limited the damage. For those wondering, his BABIP was .256 in 2023 (career .288), so that doesn't appear to be attributable to the stellar ERA.
With a career BB% of 10.9%, don't expect his command to all of a sudden improve, and while the K% will likely remain high, we should expect regression on the LOB% front. The HR/FB% of 10.9% wasn't great either and was actually below his career rate of 11.6%. All in all, it points to a higher ERA this season, especially if the free-agent pitcher latches on to a team that plays in a hitter-friendly environment, so managers should not overreach based on last year's results alone.
Gavin Williams, 3.29 ERA / 4.61 SIERA (-1.32), 165 ADP
Gavin Williams had a fine debut with Cleveland last season, going 3-5 with a 3.29 ERA, 1.26 WHIP, and 23.5% K% over 16 starts. With a 4.61 SIERA, things could have gone worse for the youngster.
The 10.7% BB% is cause for concern as well as the 9.2% HR/FB%. LOB% wasn't Snell-like, but it was well above average at 78.3%, so some of those runners coming around to score in 2024 should probably be expected. Working in his favor is a 79th percentile FB velocity (95.7) and an 85th percentile Barrel% (5.4%), with his 6-foot-6 frame and 99th percentile extension making his fastball seem faster. Stuff+ on his changeup was second-best in the league (136) only to Yu Darvish, although he uses it just around 6% of the time.
Gavin Williams, Nasty 88mph Changeup. 👌 pic.twitter.com/sqAjqFLWA2
— Rob Friedman (@PitchingNinja) July 15, 2023
Having been selected in the 2021 Draft and with only one MLB season under his belt, we don't have an extensive track record to go off of. Also, these stats should not be used in a vacuum, but for those managers trying to choose one pitcher over another, perhaps some or all of this will assist managers in their decision-making process when eyeing Williams.
Tanner Bibee, 2.98 ERA / 4.19 SIERA (-1.21), 117 ADP
Speaking of fine debuts, another 2021 draft pick for the Guardians who moved quickly through the ranks was Tanner Bibee, who finished second in AL Rookie of the Year voting. Bibee went 10-4, posting a 2.98 ERA, 1.18 WHIP, and 24.1% K% over 25 starts.
Unlike Snell and Williams, Bibee has a palatable BB% of 7.7%. His HR/FB% was better than the others as well at 8.0% and a .286 BABIP was slightly lower than the league average. Baseball Savant also shows he has a 97th-percentile Pitching Run Value. So what gives?
Well, the common theme here with these first three pitchers is the amount of runners stranded. Bibee's LOB% was 80.0%. He's also got the lowest GB% and highest FB% out of these three, so he's the least likely of the bunch to get runners out via the double play. These are reasons why his ERA will likely take a step back in 2024.
Despite the step back, it will still be a respectable ERA. ATC projections call for 3.76, but they also project that his K-BB% will improve by almost a full percentage point, which seems feasible. As mentioned above, his BB% was better than average, and other than his three starts at Triple-A, he had less than a 6.0% BB% in the minors. Projections call for an improvement to 7.3%, so there will be some trade-offs managers will have to consider for 2024.
SIERA vs. ERA Analysis: Underachievers
Taj Bradley, 5.52 ERA / 3.81 SIERA (+1.71), 264 ADP
Taj Bradley was one of the Rays' top prospects going into last season and he was given a chance to prove himself at the big league level for most of the season, recording 21 starts and 104 2/3 IP. Unfortunately, it did not go too well, with Bradley posting a 5.52 ERA, 1.37 WHIP, and an atrocious 19.2% HR/FB%.
The 8.5% BB% was right around league average, so that's not much of a concern, and he's got an excellent 28.0% K% to go with it (19.6% K-BB%). His Stuff+, which takes the batter out of the equation, came in at 110 (84th percentile) and his fastball velocity clocked in at 96.1 (85th percentile).
Taj Bradley, K'ing the Side again.
6 batters faced/6Ks. 😯 pic.twitter.com/Hhq2kXozRZ
— Rob Friedman (@PitchingNinja) June 15, 2023
Contrasting with the 'overachievers' listed above, Bradley's LOB% was 67.7%, below league average. His proclivity for giving up the long ball -- thus clearing the bases -- appears to be correlated to the low LOB%, while a .310 BABIP could somewhat attributed to the inflated ERA.
So he'll regress and everything will be fine, right? Not so fast. That is likely partially true, as he'll mature and progress as a major league pitcher (or else he won't be a major league pitcher for long at the rate he was going in 2023). But expectations for regression should be tempered, as his HR/FB% was worse in 2023 during the time he spent at Triple-A. In 10 starts there and 37 2/3 IP, he allowed nine HRs and a 23.1% HR/FB%. Throughout 12 starts at Triple-A in 2022, it was 13.9%.
Hunter Brown, 5.19 ERA / 3.70 SIERA (+1.49), 188 ADP
After allowing just two ER in 20 1/3 IP during his cup of coffee at the major league level in 2022, the following season did not treat Hunter Brown as kindly. In 2023, Brown went 11-13 with a 5.19 ERA and 1.37 WHIP over 155 2/3 IP. As atrocious as Taj Bradley's HR/FB% was, Brown's was worse at 21.0% (26 HR in 29 GS).
Brown does have a nice strikeout profile though, posting a 26.8% K% last season and no less than 25.5% at any stop in the minors. While the walks were slightly below league average in 2023 at 8.3%, it was no better than 9.7% at any stop during his time in the minors, which may portend that BB% could rise in 2024. With the high HR rate though, Brown met the same fate as Bradley, ending with a LOB% of 68.8%. The .330 BABIP surely didn't help matters.
Brown's Stuff+ checks in at 104 (75th percentile), so not as electric as Bradley's per se. However, one reason that Brown's ADP is so much higher than Bradley's (188 vs. 264) is because Brown has a 52.2% GB% (87th percentile), which means he doesn't have to rely on the K to get out of a jam since he could record a double-play ball instead. This should help keep his ERA in check going forward, assuming he can somehow reduce the number of HRs he allows as well.
FIP and xFIP vs. ERA Analysis
One last thing I wanted to take a look at was the same exercise but using FIP and xFIP. What I found was that the list of names for overachievers looked basically the same, but some names stood out on the underachievers a bit more that I wanted to specifically point out.
I won't go into detail like I did above, but when it comes draft time, it's worth knowing that these pitchers probably pitched a bit better than what their ERA suggested and could be in for a bounce-back in 2024. Yes, even Spencer Strider.
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