Kathy Iberle (Hewlett-Packard) and Sue Bartlett (IIS/STEP Technology) have developed a model to determine the ratio of software testers to software developers. The following comes from the abstract of their paper “Estimating Tester to Developer Ratios (or Not)”.

Test managers often need to make an initial estimate of the number of people that will be required to test a particular product, before the information or the time to do a detailed task breakdown is available. One piece of data that is almost always available is the number of developers that are or will be working on the project in question. Common sense suggests that there is a relationship between the number of testers and the number of developers. This article presents a model that can be used in describing that relationship. It is a heuristic method for predicting a ratio of testers to developers on future projects. The method uses the model to predict differences from a baseline project. A reader with some projects behind her will be able to come up with a rule-of-thumb model to suit her most common situations, to understand when the model might not give accurate answers and what additional factors might need to be taken into consideration.

In the paper the authors present two case studies: (1) “MergoApp”, a e-commerce website where the tester-developer ratio was 1:4, and (2)“DataApp”, a database application to replace an Excel application, where the actual tester-developer ratio was 4:8. A copy of their model can be found at Kathy Iberle’s web site (http://www.kiberle.com/articles.htm). In addition, slides for the presentation can be found here: Estimate Slides.