Reading mutation results and strengthening tests

In muttest: Mutation Testing

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)
source("engine.R")

The mutation score is only useful if you know how to read it. This article walks through three real scenarios — one for each operator class — showing what a surviving mutant looks like in the output, what it tells you about your tests, and exactly what to change.

The output table

Every muttest::muttest() run produces a table with one row per mutator/file pair:

| Column | Meaning | | ------ | -------------------------------------------------------------------------------------------- | | K | Killed — mutants that caused at least one test to fail. Your tests caught the change. | | S | Survived — mutants that passed all tests. Your tests did not notice the change. | | E | Errors — mutants that produced a parse or execution error (neither killed nor survived). | | T | Total — mutants generated for this mutator/file pair. | | % | Score — K / (K + S) × 100. |

A ✔ row means at least one mutant was killed. An x row means every mutant survived — your tests passed unchanged.

What the score tells you

| Score | Signal | | ----- | ---------------------------------------------------------------------------------------- | | 0% | Your assertions do not constrain the outcome. Tests pass no matter what. | | 1–99% | Some cases are well-tested; others are not. The survivors point at the gaps. | | 100% | Every generated mutant was caught. Your tests are robust against these specific changes. |

A 100% score does not mean there are no bugs — it means there are no untested bugs of the kinds the mutators probe. Start with the survivors; they are the actionable signal.

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shipping, mad, access # nolint

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Summary: three patterns, three fixes

| Surviving mutant | Root cause | Fix | | ----------------------- | ------------------------------------------ | ------------------------------------------------------------------------- | | Comparison (>= → >) | Boundary value never tested | Add a test that passes the exact threshold value | | Arithmetic (- → +) | Assertion checks direction, not value | Replace expect_gt/gte with expect_equal and a computed expected value | | Logical (\|\| → &&) | Symmetric inputs (both true or both false) | Add a test with asymmetric inputs (one true, one false) |

The feedback loop is:

1. Run muttest
2. Find survivors
3. Strengthen tests
4. Repeat

Start with one file and one mutator preset. Look at what survives. The survivor names the exact mutant that your tests cannot distinguish — that is your actionable signal.

muttest documentation built on May 14, 2026, 5:10 p.m.