The aim of this paper is to evaluate empirically the relationship between a new metric (Quality Assurance Indicator-Qi) and testability of classes in object-oriented systems. The Qi metric captures the distribution of the control flow in a system. The authors addressed testability from the perspective of unit testing effort. They collected data from five open source Java software systems for which JUnit test cases exist. To capture the testing effort of classes, they used different metrics to quantify the corresponding JUnit test cases. Classes were classified, according to the required testing effort, in two categories: high and low. In order to evaluate the capability of the Qi metric to predict testability of classes, they used the univariate logistic regression method.