MC/DC vs branch coverage

What is the difference?

Branch coverage asks whether each decision went both ways. MC/DC asks whether each condition inside the decision was shown, on its own, to change the result. For a decision with one condition they need the same tests. They differ when a decision joins conditions with && or ||.

One line, two answers

if (signedIn && !expired) {
  return true;
}
return false;

Two tests: a signed-in session that is not expired gets true, and a signed-out session gets false.

The if went both ways, so branch coverage is 100%. The two tests also show that signedIn matters: it is the only input that changed, and the result flipped.

Nothing shows that expired matters. Delete && !expired and both tests still pass. MC/DC is 1 of 2 conditions. A third test, signed in and expired, completes it.

How the criteria compare

Criterion	Question	Tests for `a && b && c`
Line or statement	Did it run?	1
Branch (decision)	Did the decision go both ways?	2
MC/DC	Did each condition change the result on its own?	4
Multiple condition	Was every combination tried?	8

MC/DC needs at least one more test than there are conditions. Trying every combination doubles with each condition added. MC/DC comes from avionics, where DO-178C requires it for the most critical software.

What tools report for that line

We ran the same check and the same two tests in four languages:

Tool	Result for the decision
Jest 30.5.2, Babel provider	4/4 branches
coverage.py 7.16.2	2/2 branches
`go test -cover`, Go 1.27.1	100% of statements; no branch metric
JaCoCo 0.8.15	3/4 branches
Supercov 4.3.0, all four	branches covered; `!expired` not shown

JaCoCo comes closest because it counts bytecode: javac compiles && into one conditional jump per condition, and the expired side of the second jump was never taken. That finds this gap. It still shows that each jump happened, not that a condition changed the result on its own.

Which one to use

Branch coverage is enough for code whose decisions are single conditions. Use MC/DC where decisions combine conditions: access checks, pricing rules, feature flags, validation. Use it where a standard asks for it.

Measure MC/DC on your tests

npx supercov -- npm test
npx supercov runs latest gaps

Supercov runs your existing test command and measures line, branch and MC/DC coverage for JavaScript, TypeScript, Python, Go, Java, Kotlin, Rust and Ruby. For each gap it names the condition and the missing case. The MC/DC guide walks through an example.

New to the simpler metrics? See line coverage vs branch coverage.