They have no IDEA
Did students feel free to ask questions: 4.1. Did you explain course material clearly and concisely: 3.9...
All of these numbers, of course, are cross compared to people teaching in your discipline, other institutions and even your colleagues. The forms lay you out on a bell curve so you know exactly how wonderful, mediocre or miserable an instructor you are.
Nowhere on the form does it ask if this is the first or fortieth time you've taught the course, if you are a spunky fresh-from-grad-school Boy Scout or an end-of-the-line dinosaur reeking of gin and sour disappointment. They don't weight your students with degrees of difficulty or factor in the work load of your committee assignment. There's no designated scantron bubble to note that your kid got sick and you had a fight with your spouse somewhere along mid-October. This is to say nothing of your bi-annual week of existential crisis when you wonder what the hell you have really done with your life.
And it just doesn't matter how many times you remind yourself that these gray, soulless numbers are a woefully inadequate portrait of your work, or that the IDEA forms themselves warn against making them the sole measure of teaching effectiveness. You know all this, but so what? You take one look at the numbers and that's that. The numbers are finite, complete, definitive, a revealing index of your worth as a human being.
It would be one thing if you received this information in private, but you know that the colleagues and administrators who will be judging you will also see these numbers. And you suspect that--like you--they are just as helplessly entranced by their seeming objectivity. That peer and department chair who observed your class and wrote nice things about you are fallible, biased human beings. But the numbers...
This isn't sour grapes. My own numbers are usually good and once in a while very good. Even so, it eats at me that I let them matter so much. I was standing by the faculty mailboxes yesterday and a colleague was fishing his packet of evaluation results out of his cubby. "I always feel nervous when I get these things," he muttered.
"I know," I said, and then slunk back to my office to glance over my numbers once again.