Data from the Personal Software Process (PSP) courses indicate that the PSP improves the quality of the developed programs. However, since the programs (exercises of the course) are in the same application domain, the improvement could be due to programming repetition. In this research we try to eliminate this threat to validity in order to confirm that the quality improvement is due to the PSP. In a previous study we designed and performed a controlled experiment with software engineering undergraduate students at the Universidad de la República. The students performed the same exercises of the PSP course but without applying the PSP techniques. Here we present a replication of this experiment. The results indicate that the PSP and not programming repetition is the most plausible cause of the important software quality improvements. 1. Introduction The Personal Software Process (PSP) is a software development process for the individual [1]. The PSP helps the engineer to control, manage, and improve his or her work and it is taught through a course. The students (many times software engineers) perform several programming exercises in which techniques and phases of the PSP are added as the exercises advance. For each exercise, process data are collected. Data from the courses indicate that the PSP improves the quality of the products developed [2–5]. One way used to determine this is through statistical analysis of the evolution of the results obtained by the students in each program of the course. For example, if the programs developed are of a better quality as the course progresses, then it can be statistically inferred that the PSP is responsible for the quality improvement. However, since the programs are in the same application domain, the improvement could be due to programming repetition (i.e., the learning effect). To explore the reasons for the improvements, we asked the following research question: Are the quality improvements observed in the PSP courses due to the introduction of the phases and techniques of the PSP or due to programming repetition? To investigate this we designed and performed a controlled experiment with software engineering undergraduate students at the Universidad de la República. The students performed the exercises from the course without applying the PSP techniques. This makes it possible to know if quality improves by the simple fact of programming repetition. In the context of this study, product quality is measured as defect density. The designed experiment was executed in 2012 [6], and an exact replication was
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