Rubrics are widely used in higher education to assess performance in project-based learning environments. To date, the sources of error that may affect their reliability have not been studied in depth. Using generalisability theory as its starting-point, this article analyses the influence of the assessors and the criteria of the rubrics on the assessment of two service-learning projects. A sample of 365 novice students studying for three different undergraduate degrees was evaluated by eight student assessors and two teachers at three stages of assessment. Depending on the type of project and the stage of assessment, between 19.27 and 39.55% of the total variance was attributed to the quality of the projects, 0-7.49% to the main effect of the raters, and 3.44-17.3% to the main effect of the criteria. The results demonstrated that acceptable levels of reliability (=0.70) were obtained with three raters and eight criteria or four raters and nine criteria in contexts of relative or absolute decisions, respectively.