inst/examples/mad/README.md

In muttest: Mutation Testing

Mean Absolute Deviation — Direction-Insensitive Assertion

What this demonstrates

An assertion that checks a property of the result (sign, order, non-negativity) rather than the value will be blind to arithmetic operator swaps. Both the original and the mutant satisfy the same directional constraint, so the mutant survives. Replacing the directional assertion with an exact-value assertion kills it.

The function

mean_absolute_deviation computes the average distance from a center point: mean(abs(x - center)). The subtraction x - center is the step that arithmetic operator mutators target.

The weak test

The test asserts that the result is non-negative (expect_gte(..., 0)). This checks a property of the output rather than its value.

The surviving mutant

- → + survives. Changing x - center to x + center gives a different number, but abs(x + center) is also non-negative for all inputs. For x = c(1,3,5) and center = 3:

• Original: abs(c(-2, 0, 2)) → mean = 4/3 ≈ 1.33
• Mutant: abs(c(4, 6, 8)) → mean = 6

Both are ≥ 0. The assertion cannot distinguish them.

This pattern is common in LLM-generated tests that check what is obviously true about the result (it is positive, it is numeric) rather than what is specifically correct.

The fix

Replace the directional assertion with expect_equal and a value computed by hand. Now the mutant returns 6, which is not 4/3, and the test fails.

Key rule

When an arithmetic mutant survives, replace directional assertions (expect_gt, expect_gte) with exact-value assertions (expect_equal). Compute the expected value by hand and hard-code it.

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