Basic vs. compound condition coverage

Question

I'm trying to get my head around the differences between these 2 coverage criteria and I can't work out how they differ. I think I'm failing to understand exactly what decision coverage is. My software testing textbook states that compound decision coverage can be costly (2ⁿ combinations for n basic conditions).

I would have thought basic condition coverage would be costlier.

Consider a && b && c && d && e. My understanding is that in basic condition coverage, each of these atomic variables have to have the value TRUE and FALSE in a test case for the test case to be have basic condition adequacy - that's 32 different test cases.

So what is the actual difference, and what is referred to as a "basic condition". In the example above, is a a basic condition?

Thanks.

Answer 1

Regarding terminology, I don't have a single source handy that uses the exact terms "basic condition coverage" and "multiple condition coverage". Binder's "Testing Object-Oriented Systems" says "condition coverage" and "multiple-condition coverage". Everett & McLeod's "Software Testing" says "simple condition coverage" and "compound condition coverage". But I'm certain that the first term in each case is your "basic condition coverage" and the second is your "compound condition coverage". I'll use those terms below.

Basic condition coverage means that every basic condition in the program is true in some test and false in some test, regardless of other conditions. In the following

if a && b && c
  # do stuff
else
  # do other stuff
end

there is a compound condition, a && b && c, with three basic conditions, a, b and c. It takes only two test cases, one where all basic conditions are true and one where all are false, to get full basic condition coverage. It doesn't matter that the basic conditions happen to be part of a compound condition.

Note that basic condition coverage is not branch coverage. If the compound condition were a && b && !c, those two test cases above would still achieve basic condition coverage but would not achieve branch coverage.

A less aggressively optimized set of test cases for basic condition coverage would have one test case where all three basic conditions are false and three test cases with a different basic condition true in each. That would still only be four of the eight possible combinations of basic conditions in the compound condition. The uncomfortable feeling that we're ignoring the other four is why there's compound condition coverage. That requires a test for each possible combination of basic conditions in a compound condition. In the example above, you'd need eight tests, one for each possible combination of possible values of a, b and c, to get full compound condition coverage.