Reference

MLE Translation Factors

How minor-league (and foreign-league) performance is restated as MLB-equivalent: the factor for every league, level and stat, how much data stands behind each, and how the translation performed. League rows take priority over level rows. Used on every MLE board and player page.

How MLE translation works

An MLE (minor-league equivalency) answers one question: what would this exact stat line look like against MLB pitching, in a park that favors no one? Every minor-league line on the site goes through the same six steps, in this order. The example below is an illustrative 2026 line — the numbers in the tables are the real output of the live translation code with today's factors.

1
Strip the park
Each rate is divided by half the park's effect (a hitter plays about half his games at home). Hitters: HR, 1B, 2B, 3B, K, BB. Pitchers: HR only.
2
Apply the league/level factor
Multiply by the per-component factor for that league (or the level-wide factor when a league has no row). Strikeouts rise on promotion for hitters; walks, power and contact fall.
3
Adjust for batted-ball mix
Hitters with ground-ball/fly-ball data get up to ±5% on 1B/2B/3B (ground-ball hitters post higher BABIP).
4
Adjust for age vs the level
Hitters younger than the level's average age get +1.5% per year (cap +5%); older get −1.0% per year (cap −6%). K% is not adjusted. Pitchers: no age step.
5
Rebuild the line
Ball-in-play share is recomputed from the translated K% and BB%, and wOBA, ISO and BABIP are rebuilt from the translated components.
6
Regress when projecting
The translated line is evidence, not a forecast. When seasons are blended into a projection, each stat is pulled toward the league mean in proportion to how little evidence there is (table below).

Hitter · 2026 Double-A, Eastern League (EL row), age 22, hitter-friendly park

Raw line: 520 PA · .277 AVG · 15 HR · 21.5% K · 8.8% uBB. Park factors: HR 108, 1B 101, 2B 103, K 99.

Component	Raw	Park ÷	Level factor ×	GB/FB ×	Age ×	= MLE
K% /PA	21.54%	÷0.995	×1.220	—	—	26.41%
BB% (unintentional) /PA	8.85%	÷1.000	×0.700	—	×1.042	6.45%
HBP% /PA	1.15%	—	×0.850	—	×1.042	1.02%
Singles /BIP	23.43%	÷1.005	×0.970	—	×1.042	23.56%
Doubles /BIP	8.00%	÷1.015	×0.860	—	×1.042	7.06%
Triples /BIP	0.86%	÷1.000	×0.950	—	×1.042	0.85%
Home runs /BIP	4.29%	÷1.040	×0.780	—	×1.042	3.35%

Translation: .346 → .304 wOBA (−42 points). ISO .171 → .140; K% 21.5% → 26.4%; BB% 8.8% → 6.5%.

Pitcher · 2026 Double-A, age 22-ish, park HR factor 104

Raw line: 125 IP · 25.0% K · 9.0% BB · 3.08% HR/BF.

Component	Raw	Park ÷	Level factor ×	= MLE
K% /BF	25.00%	—	×0.810	20.25%
BB% (unintentional) /BF	9.04%	—	×1.100	9.94%
HR / BF /BF	3.08%	÷1.020	×1.200	3.62%

Translation: 3.82 → 4.64 FIP. Pitchers lose strikeouts and gain walks and home runs on promotion; the pitcher factors are level-wide (the Triple-A 2023+ set differs: walks improve).

Step 6: regression to the mean, by sample size

The translated line above is what the player did, restated for MLB. A projection also asks how much of it was skill. Each stat is blended with the league mean as (rate × PA × level reliability × year weight + league mean × prior strength) ÷ (PA × reliability × year weight + prior strength). For this hitter — one Double-A season of 520 PA, Double-A reliability 0.50, most-recent-season weight 6 — the evidence counts as 1,560 effective PA, so:

Stat	Prior strength (PA)	Evidence (PA)	Weight on this line	Weight on league mean
K%	200	1,560	89%	11%
BB%	500	1,560	76%	24%
HR	500	1,560	76%	24%
Singles	1,200	1,560	57%	43%
Doubles	1,200	1,560	57%	43%

Illustration only: real projections add up to three seasons, ×2 prior strength on contact stats for players with no MLB track record (shown for singles and doubles), and extra regression for thin careers (for example ×1.8 under 200 MLB PA). Stats that stabilize quickly (K%) keep the most weight on the player's own line; contact-quality stats the least.

Definitions

Factor — a multiplier on the minor-league rate. ×0.69 on BB% turns a 10.0% minor-league walk rate into 6.9% against MLB pitching; ×1.20 on K% turns 20.0% into 24.0%.

Per PA vs per BIP — K%, BB% and HBP% are per plate appearance. Singles, doubles, triples and home runs are per ball in play (at-bats minus strikeouts), so a hitter whose strikeouts rise also loses balls in play.

League row vs level row — a league with enough promotion pairs has its own row; every other league uses its level's row. Era matters: High-A and Low-A were restructured in 2021 and Triple-A's IL and PCL merged.

Pairs and confidence — pairs = players whose minor-league season was matched to a later MLB season. High ≥1,000 · Good 300-999 · Moderate 100-299 · Low <100 · Scaled = no direct sample.

Ref. line wOBA — a fixed 600-PA minor-league line (20% K, 9% BB, 15 HR) pushed through each row with no park and no age adjustment. Kept is the share of that line's wOBA that survives.

Colors — blue-grey cells fall on translation; amber cells rise. Strikeouts are the one stat that rises for hitters; walks and home runs rise for pitchers.

Park and age are applied around these factors. Park: each component is divided by half of the park's effect, because a season is about half home games. Age: relative to the level's average age (table below), hitters get +1.5% per year younger (cap +5%) or −1.0% per year older (cap −6%) on BB, HBP and hit rates; K% is not adjusted. Ground-ball hitters get up to +5% on singles, doubles and triples and fly-ball hitters down to −5% (0.44 per unit of ground-ball rate above the level's average).

How factors vary by level

Each step down the ladder keeps less of a hitter's walks and power, and shows a bigger strikeout jump. Pitchers go the other way on walks and home runs. Hover a point for its value.

Hitters · level-wide factors (× minor-league rate)

BB%K%SinglesDoublesHome runs

Pitchers · level-wide factors (× minor-league rate)

K%BB%HR / BF

Hitter factors by league

Calibrated league rows, which take priority over the level-wide rows below. Rows with identical factors in the same era are listed together. Factors multiply the park-neutral minor-league rate.

League	Era	Pairs	Confidence	BB%	HBP%	K%	1B	2B	3B	HR	Ref. line wOBA	Kept	Note
AAA · Triple-A
International League	2021+ (IL + PCL merged)	1,617	High	0.69	0.94	1.20	0.93	0.83	0.97	0.70	.263	83%	HR fitted on the 2021-25 cohort (n=477, ratio 0.68)
IL	Through 2020	1,617	High	0.69	0.94	1.20	0.93	0.83	0.97	0.84	.270	85%
PCL	Through 2020	1,101	High	0.71	0.98	1.29	0.90	0.81	0.81	0.78	.257	80%	2021+ PCL lines use the International League row
AA · Double-A
EL	All years	329	Good	0.70	0.85	1.22	0.97	0.86	0.95	0.78	.273	86%
SL	All years	222	Moderate	0.68	0.84	1.22	0.92	0.91	0.88	0.80	.269	84%
TL	All years	242	Moderate	0.70	0.87	1.22	0.92	0.84	0.94	0.80	.266	83%
A+ · High-A
CAL	Through 2020	330	Good	0.71	0.79	1.34	0.91	0.82	0.76	0.92	.261	82%	HR capped for selection bias
CAR	Through 2020	321	Good	0.63	0.86	1.42	0.94	0.83	0.80	0.90	.257	81%	HR capped for selection bias
FSL	Through 2020	331	Good	0.75	0.93	1.42	0.92	0.96	0.81	0.88	.265	83%	HR capped for selection bias
Midwest League	2021+ (High-A)	68	Low	0.71	0.79	1.27	0.92	0.81	0.85	0.80	.261	82%
South Atlantic League	2021+ (High-A)	91	Low	0.60	0.77	1.31	0.87	0.78	0.85	0.88	.250	78%
A · Low-A
MWL	Low-A	180	Moderate	0.71	0.72	1.38	0.92	0.86	0.90	0.80	.257	80%
SAL	Low-A	198	Moderate	0.66	0.77	1.34	0.92	0.82	0.94	0.82	.257	80%
Low-A East / Low-A West	2021+ (Low-A)		Scaled	0.66	0.77	1.34	0.92	0.82	0.94	0.82	.257	80%	small pair counts; A-level calibration
A- · Short-season A
NY-Penn / NYPL / NWL	Short-season, through 2020		Scaled	0.63	0.74	1.40	0.87	0.76	0.82	0.82	.242	76%	scaled from A
ROK · Rookie / complex
PIO	Rookie		Scaled	0.61	0.72	1.44	0.82	0.72	0.72	0.75	.226	71%	scaled from A with extra K/HR adjustment
AZL / GCL	Rookie		Scaled	0.62	0.72	1.44	0.82	0.72	0.72	0.78	.228	71%	too few promotion pairs
APP	Rookie		Scaled	0.64	0.74	1.42	0.84	0.74	0.74	0.80	.234	73%	too few promotion pairs

Hitter level-wide factors and other sources

Used whenever a league has no row of its own, and for foreign, independent, college and fall leagues. Winter-league and college lines are supplemental signals; treat those rows as rough.

Level / source	Group	BB%	HBP%	K%	1B	2B	3B	HR	Ref. line wOBA	Kept
AAA	Affiliated minors	0.69	0.94	1.20	0.93	0.83	0.92	0.70	.263	82%
AA	Affiliated minors	0.69	0.85	1.22	0.94	0.87	0.92	0.78	.269	84%
A+	Affiliated minors	0.71	0.81	1.35	0.91	0.83	0.82	0.83	.257	81%
A	Affiliated minors	0.69	0.78	1.36	0.92	0.83	0.91	0.80	.256	80%
A-	Affiliated minors	0.63	0.74	1.40	0.87	0.76	0.82	0.82	.242	76%
ROK	Affiliated minors	0.62	0.72	1.44	0.82	0.72	0.72	0.78	.228	71%
APP	Affiliated minors	0.64	0.74	1.42	0.84	0.74	0.74	0.80	.234	73%
DSL	International complex	0.70	0.70	1.22	0.68	0.60	0.60	0.47	.201	63%
VSL	International complex	0.71	0.71	1.21	0.69	0.61	0.61	0.48	.205	64%
AFL	Fall league	0.91	0.93	1.07	0.92	0.87	0.87	0.78	.283	89%
IND	Other source	0.84	0.84	1.12	0.80	0.74	0.74	0.62	.245	77%
ATL_IND	Other source	0.86	0.86	1.11	0.82	0.76	0.76	0.64	.251	79%
NPB	Other source	0.93	0.93	1.04	0.91	0.87	0.87	0.83	.287	90%
KBO	Other source	0.88	0.88	1.08	0.86	0.82	0.82	0.74	.268	84%
CPBL	Other source	0.82	0.82	1.12	0.79	0.73	0.73	0.60	.241	76%
LMB	Other source	0.88	0.88	1.09	0.86	0.81	0.81	0.71	.265	83%
CUBA	Other source	0.85	0.85	1.10	0.83	0.77	0.77	0.68	.256	80%
LIDOM	Other source	0.87	0.87	1.09	0.85	0.80	0.80	0.70	.263	82%
LVBP	Other source	0.85	0.85	1.10	0.83	0.78	0.78	0.67	.256	80%
PRBL	Other source	0.87	0.87	1.09	0.85	0.80	0.80	0.69	.262	82%
PMBL	Other source	0.82	0.82	1.13	0.80	0.74	0.74	0.62	.244	76%
NCAA	Other source	0.72	0.72	1.20	0.64	0.54	0.54	0.42	.192	60%
CCBL	Other source	0.80	0.80	1.15	0.77	0.70	0.70	0.55	.232	73%

Pitcher factors

Pitchers use level-wide factors only (no per-league rows and no age step). Strikeouts fall on promotion; walks and home runs rise. Triple-A seasons from 2023 use a separate set because the automated strike zone inflated minor-league walks.

Level	K%	BB%	HR / BF	Observed K ratio	Pitchers	Used - observed
AAA	0.88	1.05	1.10	0.851	342	+0.029
AA	0.81	1.10	1.20	0.765	164	+0.045
A+	0.77	1.15	1.25	0.721	102	+0.049
A	0.74	1.18	1.30	0.731	39	+0.009
ROK	0.68	1.25	1.40
AAA, 2023+ automated strike zone	0.86	0.90	1.10

Reliability, age and baserunning scales

Reliability does not change a translated rate; it sets how much a minor-league PA counts as MLB-quality evidence when seasons are blended into a projection. Mean age is the baseline for the age adjustment.

Level	Hitter reliability	Pitcher reliability	Mean age	SB/CS x
MLB	1.00	1.00	—	—
AAA	0.75	0.70	27.2	0.85
AA	0.50	0.45	24.8	0.75
A+	0.35	0.30	22.8	0.65
A	0.25	0.20	22.0	0.55
ROK	0.12	0.10	19.5	0.45
DSL	0.04	0.03	18.5	0.45

Evidence and limits

How the factors were fitted. Promotion cohorts: players with ≥80 PA at a minor-league level and ≥40 MLB PA the same or next season, 2018-2025, each pair weighted by the harmonic mean of its two PA samples. Home-run factors are held below the fitted ratio at some levels because only hitters with power get promoted (selection bias).

Holdout check (mle_validation.py, run 2026-10-24). 165 hitters with a 2022-23 Double-A or Triple-A season (≥150 PA) and a first MLB season in 2023-24 (≥50 PA). Error = average absolute miss between the translated minor-league rate and the actual park-neutral MLB rate; bias = average signed miss (positive = predicted too high).

Factor set	Pairs	K% error (pts)	K% bias	BB% error	BB% bias	wOBA error	wOBA bias
Pre-calibration factors	165	6.22	−5.16	3.93	+3.31	.059	+0.049
Current factors	165	4.94	−1.31	2.40	+0.49	.044	+0.015

Current factors, by league	Pairs	K% error	K% bias	BB% error	BB% bias	wOBA error	wOBA bias
International League	83	4.74	−1.62	2.33	+0.63	.038	+0.017
Pacific Coast League	32	4.59	−0.12	2.37	+0.31	.060	+0.010
Eastern League	26	5.51	−2.55	2.91	+0.39	.044	+0.030
Texas League	14	4.99	−1.78	2.27	+0.93	.042	−0.001
Southern League	10	6.08	+1.25	1.93	−0.41	.048	+0.004

Limits. The current factors were fitted on 2018-2025 cohorts that include these seasons, so this is a fit check on recent data, not a clean out-of-sample test. It covers Double-A and Triple-A hitters only, and promoted players are a selected group. wOBA still runs about 15 points too high on average (+0.015): translated lines are slightly optimistic for hitters who reached MLB quickly. Lower levels (A, A+, Rookie) have fewer pairs and no holdout run here, so treat those rows as less certain.

Pitchers. The pitcher K% factors were set from 2021-24 cohorts of arms with 100+ minor-league BF followed by 400+ MLB BF. The cohort ratios sit 1-5 points below the factors used (the factors keep a small buffer for top-end arms):

Level	Pitchers	Observed K ratio	Factor used	Difference
AAA	342	0.851	0.88	+0.029
AA	164	0.765	0.81	+0.045
A+	102	0.721	0.77	+0.049
A	39	0.731	0.74	+0.009

Later graduate-cohort rechecks recorded in the source: Triple-A 2025 → MLB 2026 pitchers (n=24) confirmed the 0.90 walk factor (bias ≈ 0) and moved the Triple-A K factor from 0.83 to 0.86; the hitters' age premium was capped at +5% after +8% overshot the 21-23-year-old cohort by 28 wOBA points. Backtests of full projections against actual MLB results are on the methodology page.