Last year's Nippon Professional Baseball (NPB) import of Kodai Senga just finished an excellent rookie season, and one of the biggest stories of the hot stove season figures to be the posting of an even better SP prospect out of the same league: Yoshinobu Yamamoto. Yamamoto's numbers in the Japanese Pacific League of the NPB have been eye-popping but obviously don't translate directly to MLB. To that end, I computed a primitive projection for Yamamoto based on Major League Equivalencies and then retroactively did the same for Senga for comparison.
Method (skip to the results if the math is too dense)
Based on the guide posted here: https://homemlb.wordpress.com/2021/07/22/mle-pitching-method-full-detail/
up to step 5, MLEs were calculated based on Z-scores. Further adjustments in the original guide were skipped for now because needed data was hard to track down. The corresponding translations for Senga can be used as a sanity check.
First: determined league average RA/9 for the origin league (Pacific League 202x for both Yamamoto and Senga) - not ERA to control for official scoring and team fielding and get a "cleaner" record
Next, subtract origin league RA/9 from individual RA/9 (ex, Yamamoto 2023 RA/9=1.48 2023 PCL RA/9 = 3.58 1.48-3.58=-2.1)
Next, determine the standard deviation of the RA/9 of all pitchers in the origin league (PCL 202x), then divide the result of the above subtraction by this standard deviation. This is the pitcher's Z-score in the origin league
Now, multiply Z-score by the league standard deviation of RA/9 rates in the destination league (NL 2023)
The MLE measured in RA/9 is generated by adding the result to the league average RA/9 in the destination league (NL 2023)
This was done for the last 3 years before posting for each pitcher.
The overall projection was produced by a weighted average of yearly MLEs, weighting the most recent year 3x, year n-1 2x and n-2, 1x
Keep in mind this is measured in RA/9. To get to the more familiar ERA, I multiplied by 0.924 (the ratio of ERA to RA/9 for the 2023 NL)
Measured in ERA:
The overall results seem reasonable for Senga, if a bit lower than his FIP of 3.64 or xFIP of 3.77 (using fielding independent numbers to keep things as neutral as possible, particularly as Senga ran a fairly large gap between this and his actual ERA). Multiplying Yamamoto's initial MLE by the ratio of Senga's 2023 xFIP to projection gets us to 3.10.