Phase 4: Mutation Testing & Behavioral Verification · 50 min · mutmut · cosmic-ray · pytest

Interpreting Mutation Scores

A 60% mutation score means 40% of your code's behavior is unverified. Triage every survivor.

Real-world application: Engineers who can triage surviving mutants (kill vs equivalent vs timeout) demonstrate the analytical skills needed to improve test suites systematically.

What you will learn

Interpret mutation scores: what 60%, 80%, and 100% actually mean
Classify surviving mutants as killable, equivalent, or timeout
Apply the triage workflow: for each surviving mutant, decide kill/equivalent/timeout
Write tests that kill surviving mutants to improve the mutation score

Introduction

Interpreting Mutation Scores

Objective

Run the mutation score triage workflow on 20 mutants (12 killed, 5 surviving, 2 equivalent, 1 timeout). Classify each surviving mutant, calculate how many tests to write to reach 80%, and generate an improvement plan.

A 60% mutation score means 40% of your code's behavior is unverified. Triage every survivor.

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You've read the first 2 sections. The rest of this lesson covers The Problem, The Concept, Build, Check Yourself, Key Terms & Next — plus a hands-on lab, quiz, and project artifact.

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