Measuring Success

You don't want your customers to encounter serious bugs. Nevertheless, there will always be bugs in the software you ship. You can't find every bug, and you can't even afford to fix all the bugs that you do find. All you can do is work hard to find the most severe bugs quickly and efficiently, so that there are no major customer issues.

Thus I can sum up the mission of Quality Assurance in two words:

No Surprises.

Your users are going to find lots of little things, guaranteed. Minor UI glitches, bad help system content, minor incompatibilities with 3rd-party software -- none of that should be surprising. The testing effort is a success if all the defects reported by your customers are either minor or already in the bugbase.

Conversely, your testing effort was lacking if your users report serious issues not already in the bugbase. When that happens, you need to take a hard look at your methodology and figure out exactly how those bugs escaped notice.