The body effect parameter comes into play for analog CMOS design when the source and substrate is not connected to the same node but have a reverse bias voltage across them. It affects the threshold voltage of the MOSFET and thereby is critical to the operation of the MOSFET. Signal Processing Group has released a simple calculator to evaluate this parameter given the substrate doping and the gate capacitance. The solution is based on a first order model and is primarily useful as a starting point for design. Please visit the Signal Processing Group website for this and other very useful information.