Abstract
This paper presents the results of a study that aimed to compare the traditional concurrent think-aloud protocol with the co-participation method to determine the benefit of adding an additional participant to the testing session. The two methods were compared through an evaluation of a library website, and their relative validity and utility were measured using four points of comparison: overall task performance, test participants' experiences, quantity and quality of problems discovered, and the cost of employing each method. The results of the study show significant differences between the performances of the two types of testing methods. The co-participation method was evaluated more positively by users and led to the detection of a greater number of minor usability problems. This method, however, was found to require a greater investment of time and effort on the part of the evaluator in comparison to the classical method. This study found no difference between the methods in terms of task performance.