Seven misconceptions of MC/DC #3: Getting 100% MC/DC means you've tested 100% of your requirements

If you’ve managed to get to 100% MC/DC, does that mean you’ve tested 100% of your requirements?

Remember that 100% MC/DC means that all of the Boolean decisions have been exercised and that entry and exit of each function has been covered. Requirements will typically govern input-output relationships for the whole function. For example, consider the function:

int perc_mix( int perc, int p0, int p100 ) {
    int result;

    if( perc < 0 ) {
        result = p0;
    } else if( perc > 100 ) {
        result = p100;
    } else {
        result = ((p0 * perc) / 100) + ((p100 * ( 100-perc )) / 100);
    }
    return result;
}

And its requirements:

1: at perc=100 or over, return p100

2: at perc=0 or below, return p0

3: at perc between 0 and 100, return a linear weighted mean of p0 and p100, with the weight p0=1, p100=0 at perc=0 and p0=0, p100=1 at perc=100.

 A test suite of perc=-50, perc=0, perc=100, perc=150 achieves full MC/DC but does nothing to exercise requirement 3. (Hopefully the tests have been constructed with differing values of p0 and p100, so that the error in the else-branch of the implementation can be found.) To demonstrate that you’ve tested 100% of your requirements, you need to show:

• That you have a test for 100% of your requirements;
• That 100% of your tests actually test a requirement;
• That every test passes.

Notice that these can be achieved by review – MC/DC is not playing a part here.

So what is MC/DC doing? Once you’ve met the above conditions and you also achieve 100% MC/DC, you will know that your tests have exercised all of the decisions and conditions in your application.