Modified Condition Decision Coverage (MCDC)

Modified Condition / Decision Coverage (MC/DC) is a strong white-box testing criterion often used in safety-critical systems (like aviation or medical software). It ensures very thorough testing by requiring more than just simple condition coverage.

To achieve MC/DC, you must satisfy:

• Each entry and exit point in the program must be invoked at least once
• Each decision (e.g., an if statement) must evaluate to both True and False at least once
• Each condition (e.g., parts of a compound condition like a > 10 or b < 20) must evaluate to True and False
• Each condition must independently affect the outcome of the decision

MC/DC requirement

MC/DC requires us to:

• Make the decision evaluate to both True and False
• Have each condition evaluate to both True and False
• Show that each condition independently affects the result of the decision

Example

a = ...
b = ...
if (a > 10 or b < 20):
    return False
return True

This is a compound decision with two conditions:

• Condition 1: a > 10
• Condition 2: b < 20

The test suite to satisfy MC/DC:

• Decision takes both True and False values
  • True → T1, T3, T4
  • False → T2
• Each condition takes both True and False
  • a > 10: True (T1, T4), False (T2, T3)
  • b < 20: True (T3, T4), False (T1, T2)
• Each condition independently affects the decision
  • For a > 10: Compare T1 vs T2:
    • T1: a > 10 = True, b < 20 = False → Decision = True
    • T2: a > 10 = False, b < 20 = False → Decision = False
  • The only change is in a > 10, and it changes the decision
  • For b < 20: Compare T3 vs T2:
    • T3: a > 10 = False, b < 20 = True → Decision = True
    • T2: a > 10 = False, b < 20 = False → Decision = False
  • The only change is in b < 20, and it changes the decision

Test	a	b	a > 10	b < 20	Decision (a > 10 or b < 20)	Return
T1	11	30	True	False	True	False
T2	9	30	False	False	False	True
T3	9	10	False	True	True	False
T4	11	10	True	True	True	False

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