What makes a city first or second or third tier?

Someone asked me today: "On what criteria or data is used that would consider a city a second tier city versus a first tier city. I know that most of us seasoned planners know that when you say New York, San Francisco, Chicago, we think “first tier” and Minneapolis, Denver, San Antonio “second tier,” but what makes it 1st or 2nd tier?"

Ah, the age-old tier question, which, unfortunately, is one without any really great answers that I know of. According to the Convention Industry Council APEX Glossary, a second-tier city is “A city where the space limitations of the convention center, the hotels, or the air lift, make the city more appropriate for smaller meetings and events.” But under that definition, New York would be second tier, which it generally isn’t thought to be. They don’t have a definition for first tier.

I dug around a bit and found a couple of articles that might be useful (here and here), but I really couldn't answer her question. And what is second-tier to some is third-tier to others.

If you have a more scientific way of looking at what tier a city might fall into, I'd love to hear it either via e-mail or drop a comment below. Thanks!