"Perceived exertion" measures the same thing that a heart rate monitor is supposed to measure - physical activity intensity. And it can be done on a scale of 1-10 or 1-4.

What I was trying to explain, just from my own experience, is that measuring your heart rate does nothing if you don't use it to pace yourself. And it's easier to pace physical activity intensity in a spinning class with an instructor who uses physical activity intensity as his or her benchmarks for the class.

There are a lot of instructors who simply tell the class what resistance they should be at (1-10), whether they should sit or stand, and what cadence to achieve. Unfortunately that's worthless if you're not in the same shape as the instructor.

So again, from my personal experience, I've found it helpful to try a variety of instructors, and I personally particularly like instructors who measure how hard you're supposed to work as opposed to what resistance you're supposed to be at.