The vibration of bearings are divided into 4 time periods. These periods are named as “Nstep”, “N1step”, “N2step”, and “N3step” by marching from present to past. The value presented for each of these periods are the average of vibration in respective time interval. The differences between the value of “Nstep” and “N1step”, “N2step”,and “N3step” determines the StepChange status.