The undergraduate physics laboratory courses at the two Amsterdam universities are process type labs, where students are trained to develop scientific reasoning skills, experimental skills and communication skills. In these process labs students conduct open inquiry experiments, wherein they mobilize their knowledge, relate theoretical concepts to the practice of doing physics and communicate about ideas and findings with fellow students and lab assistants. Experimental skills, performed by students, are classified according to three lines: the nature of the task, the complexity of the task and the level of guidance provided by the lab assistant to fulfill the task. This classification supports the lab assistant to choose the proper type of feedback during the guidance and helps the lab assistant to improve the formative and the summative assessment of the students. With this setup it is possible to teach complex skills to a large number of students.