A Bode plot gives the best idea of frequency response in a visual format. There are far too many frequencies and possible problem areas (peaks, phase, etc) to give frequency response a single number, such as is done with distortion.
Basicaly this depend on stray capacitances that exist in the circuit.But I can not find the expression and dependency on frequency.This is also important because if this is due to stray capacitances ,it is unexpected result.