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. (Editors note: Click here to read another paper on this topic, by Cem Kaner, Elisabeth Hendrickson, and Jennifer Smith-Brock.)