Scientific abilities rubrics developed at Rutgers University are tools for both formative assessment and student self-assessment. They consist of tables, each representing a broad scientific ability (for example, "to collect and analyze data"); the listing of sub-abilities (for example, "independent and dependent variables are identified"); and the criteria to assess to which degree the sub-ability has been developed. The rubrics were developed to assess student work in the project laboratory, a project-based course where groups of students solve open-ended experimental physics problems. They submit a report in the form of a web page, which is evaluated and returned to the students with feedback on what needs to be improved. These iterations are repeated until the report is deemed acceptable. This article presents authors' experience with the rubrics, an analysis of which abilities appear to be the most difficult to evolve, and the development of a new rubric for the web report. Introduction of the rubrics reduced the workload of the graders, while increasing the quality of the reports. Based on these results we conclude that using scientific abilities rubrics is a very efficient way of assessing experimental project-based work.