Modified condition/decision coverage (MC/DC) is a code coverage criterion used in software testing.
Overview
MC/DC requires all of the below during testing:
Each entry and exit point is invoked
Each decision takes every possible outcome
Each condition in a decision takes every possible outcome
Each condition in a decision is shown to independently affect the outcome of the decision.
Independence of a condition is shown by proving that only one condition changes at a time.
MC/DC is used in avionics software development guidance DO-178B and DO-178C to ensure adequate testing of the most critical (Level A) software, which is defined as that software which could provide (or prevent failure of) continued safe flight and landing of an aircraft. It is also highly recommended for SIL 4 in part 3 Annex B of the basic safety publication and ASIL D in part 6 of automotive standard ISO 26262.
Additionally, NASA requires 100% MC/DC coverage for any safety critical software component in Section 3.7.4 of NPR 7150.2D.
Definitions
Condition A condition is a leaf-level Boolean expression (it cannot be broken down into simpler Boolean expressions). Decision A Boolean expression composed of conditions and zero or more Boolean operators. A decision without a Boolean operator is a condition. A decision does not imply a change of control flow, e.g. an assignment of a boolean expression to a variable is a decision for MC/DC. Condition coverage Every condition in a decision in the program has taken all possible outcomes at least once. Decision coverage Every point of entry and exit in the program has been invoked at least once, and every decision in the program has taken all possible outcomes at least once. Condition/decision coverage Every point of entry and exit in the program has been invoked at least once, every condition in a decision in the program has taken all possible outcomes at least once, and every decision in the program has taken all possible outcomes at least once. Modified condition/decision coverage Every point of entry and exit in the program has been invoked at least once, every condition in a decision in the program has taken all possible outcomes at least once, and each condition has been shown to affect that decision outcome independently. A condition is shown to affect a decision's outcome independently by varying just that condition while holding fixed all other possible conditions. The condition/decision criterion does not guarantee the coverage of all conditions in the module because in many test cases, some conditions of a decision are masked by the other conditions. Using the modified condition/decision criterion, each condition must be shown to be able to act on the decision outcome by itself, everything else being held fixed. The MC/DC criterion is thus much stronger than the condition/decision coverage.
Criticism
It is a misunderstanding that by purely syntactic rearrangements of decisions (breaking them into several independently evaluated conditions using temporary variables, the values of which are then used in the decision) which do not change the semantics of a program can lower the difficulty of obtaining complete MC/DC coverage.
This is because MC/DC is driven by the program syntax. However, this kind of "cheating" can be done to simplify expressions, not simply to avoid MC/DC complexities. For example, assignment of the number of days in a month (excluding leap years) could be achieved by using either a switch statement or by using a table with an enumeration value as an index. The number of tests required based on the source code could be considerably different depending upon the coverage required, although semantically we would want to test both approaches with a minimum number of tests.
Another example that could be considered as "cheating" to achieve higher MC/DC is:/* Function A */
void function_a (int a, bool b, bool c, bool d, bool e, bool f) { if (a == 100) { if (b || c) // statement 1
if (d || e || f) // statement 2 } }/* Function B */
void function_b (int a, bool b, bool c, bool d, bool e, bool f) { bool a_is_equal_to_100 = a == 100 ; bool b_or_c = b || c ; bool d_or_e_or_f = d || e || f ;
if (a_is_equal_to_100) { if (b_or_c) // statement 1