# Metrics

Reference for every stat the app can plot, plus the **Min PA / IP** knob that filters per-season data before the curve is fit. All formulas use the abbreviations as they appear in the underlying MLB Stats API.


# Batting metrics

For batters, the **weight column** in the weighted least-squares fit is PA (plate appearances). Anchor points below are full-season values for full-time players.


## AVG -- Batting average

Hits per at-bat:

    AVG = H / AB

A decimal between 0 and 1.

| Range   | Read               |
|---------|--------------------|
| `.300+` | Excellent          |
| `.275`  | Solid regular      |
| `.250`  | Average MLB hitter |
| `.220`  | Below average      |


## OBP -- On-base percentage

Fraction of plate appearances where the batter reached base by hit, walk, or hit-by-pitch:

    OBP = (H + BB + HBP) / (AB + BB + HBP + SF)

Where `SF` = sacrifice flies. OBP excludes errors and fielder's choices.

| Range   | Read          |
|---------|---------------|
| `.400+` | Elite         |
| `.360`  | All-Star      |
| `.330`  | Solid regular |
| `.300`  | Below average |


## SLG -- Slugging percentage

Total bases gained per at-bat. A double counts as 2, a triple as 3, a home run as 4:

    SLG = (1B + 2·2B + 3·3B + 4·HR) / AB

| Range   | Read          |
|---------|---------------|
| `.500+` | Power hitter  |
| `.450`  | Strong power  |
| `.400`  | Average       |
| `.350`  | Limited power |


## OPS -- On-base + Slugging

The sum of OBP and SLG. The single most-used summary of offensive value:

    OPS = OBP + SLG

| Range   | Read          |
|---------|---------------|
| `.900+` | All-Star tier |
| `.800`  | Above average |
| `.700`  | Average       |
| `.600`  | Below average |


## HR -- Home runs

The count of home runs in a season -- a raw total, not a rate stat. The curve fits this against age directly.

| Range | Read (full season)  |
|-------|---------------------|
| `40+` | Elite power         |
| `30`  | Power hitter        |
| `20`  | Full-time regular   |
| `10`  | Part-time / contact |


## ISO -- Isolated power

Pure extra-base power, with singles' contribution removed:

    ISO = SLG − AVG

A batter who hits only singles has SLG = AVG and therefore ISO = 0.

| Range   | Read         |
|---------|--------------|
| `.250+` | Elite power  |
| `.200`  | Power threat |
| `.150`  | Average      |
| `.100`  | Light power  |


# Pitching metrics

For pitchers, the weight column is IP (innings pitched). All per-9 rates multiply by 9 to extrapolate to a full game's worth of innings.


## ERA -- Earned run average

Earned runs allowed per 9 innings:

    ERA = 9 × ER / IP

Where `ER` excludes runs that scored after a fielding error. **Lower is better.**

| Range   | Read          |
|---------|---------------|
| `<2.00` | Cy Young tier |
| `3.00`  | All-Star      |
| `4.00`  | Solid regular |
| `5.00+` | Below average |


## WHIP -- Walks + Hits per Inning Pitched

Baserunners allowed per inning:

    WHIP = (BB + H) / IP

**Lower is better.**

| Range   | Read          |
|---------|---------------|
| `<1.00` | Elite         |
| `1.20`  | All-Star      |
| `1.30`  | Solid regular |
| `1.40+` | Below average |


## K/9 -- Strikeouts per 9 innings

    K/9 = 9 × K / IP

**Higher is better.**

| Range | Read            |
|-------|-----------------|
| `11+` | Power pitcher   |
| `9.0` | Strong          |
| `7.0` | Average         |
| `5.0` | Contact pitcher |


## BB/9 -- Walks per 9 innings

    BB/9 = 9 × BB / IP

**Lower is better** (better control).

| Range  | Read          |
|--------|---------------|
| `<2.0` | Elite control |
| `2.5`  | Strong        |
| `3.5`  | Average       |
| `5.0+` | Wild          |


## HR/9 -- Home runs per 9 innings

    HR/9 = 9 × HR / IP

**Lower is better.**

| Range  | Read        |
|--------|-------------|
| `<0.8` | Elite       |
| `1.0`  | Strong      |
| `1.2`  | Average     |
| `1.5+` | Homer-prone |


# Min PA / IP per season

The **Min PA / IP per season** knob in the sidebar drops per-season rows from the curve fit when they fall below a minimum playing-time threshold. The unit follows the **Position** toggle:

- **PA (plate appearances)** when *Position* is *Batter*.
- **IP (innings pitched)** when *Position* is *Pitcher*.

It exists because a 50-PA September call-up or a 20-IP injury year carries almost no signal about a player's true ability. A weighted quadratic fit will still treat such a row as a data point (lightly weighted, but present), and very-low-weight seasons can drag the fit when they're outliers. Setting a minimum drops those rows entirely.


## Common thresholds

| Value | What it represents |
|----|----|
| `502 PA` | The official batting-title qualification (3.1 PA × games scheduled). |
| `162 IP` | The official ERA-title qualification (1 IP × games scheduled). |
| `300 PA` / `100 IP` | Typical "full-time regular" lower bound. |
| `100` | The default; keeps most non-trivial seasons. |
| `0` | Disables the filter -- every season counts toward the fit. |

For a player whose role changed substantially over time (starter-turned-reliever, regular-turned-bench), tuning this knob up to focus on their prime role often gives a cleaner curve. For a young player with only a few full seasons, tune it down to include near-rookie years.


# See also

- [Modeling approach](modeling.md) -- how the quadratic fit uses these metrics as the response variable, and how weights are applied.
- [Methods](methods.md) -- worked example using OPS for Mickey Mantle, with the centered-form interpretation.