> ## Documentation Index
> Fetch the complete documentation index at: https://mehen.ophi.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Tateishi RS + Jōyō grade

> Japanese readability scores built from script composition without a morphological analyzer.

mehen's Tier-0 Japanese readability score is the **Tateishi simplified RS** (Tateishi, Ono & Yamada
1988\). It needs only sentence boundaries and script-run features — both computable without a tokenizer.

## Tateishi RS

Simplified 6-variable form:

```text theme={null}
RS = −0.12·ls − 1.37·la + 7.4·lh − 23.18·lc − 5.4·lk − 4.67·cp + 115.79
```

Calibrated so mean ≈ 50, SD ≈ 10. **Higher = easier**. Mehen emits this as `tateishi_rs` with sanity
guards:

* Refuse when `hiragana_ratio > 0.90` (the formula is gamed upward).
* Refuse when character count \< 300.

## Jōyō grade proxy

The 2,136-character 2010 Jōyō list maps each character to a grade 1–8 (1–6 elementary, 7 = secondary
Jōyō, 8 = non-Jōyō `hyōgai`). Ships as a \~6 KB static table behind `--features japanese-jouyou`:

```text theme={null}
jouyou_grade_mean = mean(grade(c) for each kanji c)
hyougai_ratio     = non_jouyou_kanji_chars / kanji_chars
```

`jouyou_grade_mean` is a direct school-grade analogue to Flesch-Kincaid: \< 3 indicates elementary
reading; > 6 indicates high-school+ technical prose.

## Higher-tier formulas (gated)

Behind `--features japanese-morph` (Lindera + IPADIC) or `--features japanese-unidic` (Vibrato +
UniDic):

* **Shibasaki & Hara** — adds bunsetsu and morphological inputs.
* **Lee & Hasebe jReadability** — modern web-corpus-trained formula.
* **Obi / Obi2** — Japanese textbook-grade analogue.
* **Mizuno / Goda** — alternative readability work.

JLPT bands — N5–N1 word and kanji bands — are optional behind `--features japanese-jlpt`
(\~300 KB).

## References

* Tateishi, K., Ono, Y. & Yamada, H. (1988). *A Computer Readability Formula of Japanese Texts for
  Machine Scoring.* Proceedings of COLING-1988: 649–654.
  [ACL Anthology](https://aclanthology.org/C88-2132/).
* 文化庁 (Agency for Cultural Affairs of Japan, 2010). *常用漢字表.* (Jōyō kanji list, 2010 revision.)
  [Official notice (PDF)](https://www.bunka.go.jp/kokugo_nihongo/sisaku/joho/joho/kakuki/14/tosin02/index.html).
* Lee, J. & Hasebe, Y. (2017). *jReadability — a web-based Japanese text-readability indexing system.*
  [jReadability](https://jreadability.net/sys/en).
* Sato, S., Matsuyoshi, S. & Kondoh, Y. (2008). *Automatic Assessment of Japanese Text Readability
  Based on a Textbook Corpus.* Proceedings of LREC 2008.
  [LREC PDF](https://aclanthology.org/L08-1064/).

## See also

* [Japanese script composition](/metrics/markdown/prose/japanese-script-composition) — provides the
  inputs.
* [JTF rules](/metrics/markdown/prose/jtf-rules) — Japan Translation Federation conformance.