---
title: "Evidence Coverage"
description: "Per-section structural support density across links, code, tables, and diagrams."
keywords: ["evidence coverage", "documentation", "anchors", "support"]
---

**Evidence Coverage** measures structural support for each section. Where
[Repository Grounding](/metrics/markdown/repository-grounding) is a document-level signal, Evidence
Coverage is per-section, so one well-linked section cannot hide many unsupported ones.

## Formula

```text
anchor_density_s   = evidence_anchors_s / max(1, W_s / 250)
section_evidence_s = sat(anchor_density_s; 0.2, 1.5)

EvidenceCoverageScore =
    0.5 · mean(section_evidence_s)
  + 0.5 · p25(section_evidence_s)
```

## What is an "evidence anchor"

For each section, mehen counts:

- Resolved internal anchors.
- Resolved relative repository links.
- Code fences with explicit language tags.
- Diagram blocks (Mermaid, GraphViz, etc.).
- Table headers.
- Footnote references.

These are normalized against the section's word count (per 250-word window).

## Why the 25th-percentile term matters

The mean alone hides skew: one section with 10 anchors and four sections with 0 anchors averages 2,
which looks fine. The 25th-percentile term ensures the score reflects the **worst supported** sections.

## How downstream metrics use it

Evidence Coverage feeds [DMI](/metrics/markdown/dmi) indirectly via Filler / Lazy Risk and is reported
in the [PR comment](/guides/pr-comment-design) drill-down.

## References

- Daugherty, S. R. & Krueger, K. R. (1991). *The 90-10 rule of unequal coverage and its
  ramifications for measurement.* Psychological Reports 68(3) — motivation for percentile-based
  reporting (the 25th-percentile term that prevents one well-anchored section from hiding many
  unsupported ones).
  [DOI](https://doi.org/10.2466/pr0.1991.68.3.939).
- Pirolli, P. & Card, S. (1999). *Information Foraging.* Psychological Review 106(4): 643–675 —
  source for the "evidence anchor" concept (each link, code block, or table is a foraging cue).
  [DOI](https://doi.org/10.1037/0033-295X.106.4.643).

## See also

- [Repository Grounding](/metrics/markdown/repository-grounding) — document-level analogue.
- [Filler / Lazy Risk](/metrics/markdown/filler-lazy-risk) — uses unanchored prose mass as an input.
