1. A method for determining a magnitude of a change in a cell population comprising at least one cell, said method comprising the steps of a) sequentially acquiring at least two images of the cell population b) comparing at least a part of the at least two images, thereby obtaining at least one difference image c) computing a parameter from the at least one difference image, wherein said parameter provides information of the magnitude of a change, and d) based on said computed parameter determining the magnitude of the change.