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, wherein the parameter computed from the difference image is selected from the group of Sum of absolute values for pixels in the difference image; Mean absolute value for pixels in the difference image; Median absolute value for pixels in the difference image; Sum of squared values; Mean squared value for pixels in the difference image; Median of squared value for pixels in the difference image; Variance of values for pixels in the difference image; Standard deviation of values for pixels in the difference image; Values for different percentiles in the histogram of the image; Difference Image minimum and maximum values; and Range or variance of histogram or another parameter derived from a combination of one or more of these parameters.