I've always wondered how employers measure "market rate". If there were a reliable public source of market rate, negotiations would be much simpler, and they aren't. And presumably the managers themselves aren't actively interviewing at other companies to see what they'd get offered.

So I think the only good source of market rate is people (either candidates or current employees asking for counteroffers), and if you're poor at being competitive for recruits on all axes other than money, or at making it safe for employees to interview and tell you about their offers, you have poor information.