In one aspect, the present invention relates to a system 100 for automated cellular assay data analysis. The system 100 comprises a virtual assay module (VAM) 115 operable to generate simulated images of cell responses to one or more stimuli. The system 100 also comprises a comparator module 116 operable to compare the actual and simulated images, and an analysis module 117 operable to quantify the differences between phenotypes represented by the actual and simulated images. Various aspects and embodiments of present invention may account for stochastic variations in the response of single cells, to provide additional useful information relating to, for example, toxological effects and/or for use as part of a feedback mechanism to refine dynamically a virtual assay model such that it is not limited by way of there being only inadequate static fitting expressions available.