IMD stands for inter-modulation distortion and is a measurement of the first set of unwanted sidebands on a PSK31 transmission to the desired ones. It is then measured in decibels. Generally, a -30dB is considered a excellent IMD, while -20dB is a poor transmission.
Some operators think that a higher reading on the watt meter means a better signal. However, that is not the case. Rather, a clean signal is to be desired. One easy way to check the quality of your signal is to look at the automatic gain control meter on your radio. For the optimum signal, it should read near zero. Better yet, have the receiving station check you IMD (most software programs have this feature). For the most accurate measurement, you can purchase a IMD meter.
This being the case, a 35W signal with an IMD of -30dB is better than a 100W signal with a -19dB IMD. This shows that it is important to tune your transmitter for maximum quality, not maximum power.
To improve your IMD, try adjusting volume levels on the computer sound panel, sound card interface volume, as well as transmitter settings.