In the week after J.D. Power released the results of its 2008 Initial Quality Survey (IQS), I’ve come across quite a few forum posts that went like this: “Great news: our car did very well. But why is BMW’s quality so bad?”

Yes, BMW again scored below the average on this year’s IQS. But this isn’t because the cars suffer from many defects. What people easily forget (if they ever knew it): in recent year’s J.D. Power’s IQS has measured two very different things: “design quality” and “mechanical quality.” The latter measures what most people think of when they hear “quality”: things that break and needed to be fixed. “Design quality,” on the other hand, concerns problems with driveability and usability that are designed into the car: the smoothness of the engine, the ease of operating the audio system, and so forth. These aren’t things that can be repaired.

BMW’s low score is largely due to one thing: the make’s iDrive control system. Auto reviewers have been criticizing it from Day One, and it seems owners don’t much care for it, either.

But it’s a mistake to infer from complaints about iDrive that BMWs are unreliable. Of course, this wouldn’t happen if J.D. Power didn’t combine the two areas into a single score.