A subtraction technique, continuous phase shift subtraction, is presented and compared both theoretically and experimentally to some Digital Speckle Pattern Interferometry (DSPI) techniques for vibration measurements. The method is found to be an improvement to other techniques in rough environments due to low sensitivity to disturbances. Continuous phase shift subtraction is a technique for finding resonance frequencies and corresponding modal shapes which is cost effective, robust, stable and easy to handle, i.e. a technique suitable for industrial use. The comparison assumes a DSPI system for out-of-plane measurements and the described methods are all based on the time-averaged technique with additional digital image processing.