path: root/usth/ICT2.7/P4L3 White-Box Testing Subtitles/22 - MC DC Coverage - lang_en_vs4.srt

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And this criterion is called Modified Condition/Decision Coverage,

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also called MC/DC. This criterion is very important because

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it is often required for safety critical applications.

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For example, the FAA, the Federal Aviation Administration, requires

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for all the software that runs on commercial

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airplanes to be tested according to the Modified Condition/Decision

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Coverage. So what is the key idea behind the

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MC/DC criterion? It is to test only the important

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combinations of conditions instead of all of them, and limit

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the testing cost by excluding the other combinations. And the way

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in which it works is by extending branch and decision

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coverage with the requirement that each condition should affect the decision

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outcome independently. So let's see what this means with an

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example that will also show you how you can reduce the

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number of combinations in this way. I am going to

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show you an example of how MC/DC works using this predicate

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which consists of three conditions. a, b, and c, which are all

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in and, so the overall predicate is a and b and c.

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The first thing I'm going to do is to show you how

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many test cases we will need to satisfy the multiple condition coverage

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for this simple predicate. Which means, how many test cases we will

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need to test all the possible combinations of true and false values

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for these conditions. So I'm going to populate this table. And as you

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can see, at the end we have eight test cases. Each test case

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tests a different combination of values for a, b, and c.

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I'm also showing, for each test case, the outcome of the overall

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predicate. So, for example, if we look at the first one, the

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first test case, will be such that a is true, b is

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true, and c is true. And therefore, the overall outcome of

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the predicate is true. Now lets consider the first condition, a. As

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I said a minute ago, what we want to test are the

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important combination. Which are the comibatinos

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in which a single condition independently

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effects the outcome of the overall predicate. So if we consider a

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and we look at this possible set of this cases. Let's try to

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find two test cases such that the only difference between the two

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test cases is the value of a, and the overall outcome of the

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predicate is different. If we look at the table, we can see

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that this is true for test cases one and five. If we look

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at these two cases, we can see that the overall of the predicate

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in the two cases is true and false, and that the only difference

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between the value of the conditions in the value of a. So

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these test cases satisfy exactly what we wanted. There are two test

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cases in which the value of a independently decides the overall value

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of the predicate. What we do, therefore, is to add these first

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two test cases to our set of tests down here. Now let's

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focus on b and let's try to find two test cases such

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that the value of b is the only value that changes between

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the two test cases, but the overall value of the predicate is

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different, the same thing we did for a. And in this case,

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we can see that if we select test case number one, and test

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case number three, we have exactly that situation. b is true in the

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first case, false in the second one, a and c don't change, but

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the overall value of the predicate changes. And now you can notice

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something else. That even though we selected two test cases, tested two values,

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one we already had. So, we only need three test cases overall to

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test a and b according to MC/DC. Now, let's look at our last

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condition, c. At this point, we know the game, so we

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just have to look for two test cases that satisfy our

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requirements. And in this case, one and two are suitable candidates.

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And once more, because we already have one, we just have to

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add two to our list. So as you can see from

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this example, we went from having eight test cases needed to cover

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all the possible combinations of conditions to only four test cases

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to satisfy the MC/DC criteria. So let's see where MC/DC stands in

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our substantion hierarchy. This is what we had so far

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in the hierarchy and this is where the MC/DC criterion will

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stand. MC/DC criterion is stronger than branch and condition coverage.

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Why? Because it requires every single condition to be true and

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false. And therefore, this section was a condition coverage criteria.

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And it also requires every predicate to be true and false

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and therefore, this section is branch coverage. And in addition,

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it's got the additional requirements that the true and false values,

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all the conditions have to also decide the overall

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value of the predicate. So it's stronger. Which is more

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thorough than branch and condition coverage and, as usual,

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also stronger than branch coverage, statement coverage, and condition coverage.