The correct answer in this case is, “Wait, I thought this was a software engineering book, not a
mathematics text. You didn’t say I was going to have to solve word problems!”
Of course you don’t really know how many bugs are in the application, but the Lincoln index gives
you a guess. In this example, the Lincoln index is 15 × 13 ÷ 5 = 39.
More generally, if two testers find E1 and E2 errors respectively, of which S are in common, then the
Lincoln index is given by the following equation: