We have established that a Poisson model describes quite well the finding of usability problems in user testing and heuristic evaluation. Such a model can therefore be used to predict the eventual number of problems that will be found by a usability study even as the study is in progress. Further work remains to be done to assess the preciseness of these predictions, but similar models have been successfully applied to the related problem of determining when to stop testing software for programming bugs [1] [2].