Originally Posted by jgwilliams

If you don't want to change the rear derailleur then you need to find the pull ratio and get some shifters that match. The pull ratio is a measurement of how much cable is pulled for a given movement of the derailleur. Unfortunately manufacturers love to mess with the ratio from one year to the next - ostensibly to improve performance. As long as the shifter pull ratio matches that of the derailleur then it will work. The same is true for the front derailleur, of course, but it's not quite so critical.