Just to give an example, as EXCAP232 noted, the control throw off a servo is non-linear because the servo arm is moving on a circular path.

If you have a computer transmitter that lets you set exponential, you can set it to -18 to -20% exponential and have a servo output that is linear. Note that you put exponential in as a negative % (at least on my equipment). If you go to a +%, you are sharpening up control right around the center of the stick travel - not normally advised.

I had a friend I was instructing once who just couldn't keep from making big throws of the aileron stick. Finally, set his transmitter up with about -50% exponential and finally got him where he solo'ed his trainer.

Take care,