Science teachers regard practical work as important and many claim that it helps students to learn science. Besides theoretical knowledge, such as concepts and formulas, practical work is considered to be an integral and basic part of science education. As practical work is perceived and understood in different ways, comparing the results between classes and schools is difficult. One way of making the results comparable is to develop systematic inquiries to be assessed in national large-scale tests. However, introducing similar testing conditions in a laboratory environment is not always possible. Although the instructions and assessment guides for such tests are detailed, many obstacles need to be overcome if equality in the overall test situation is to be achieved. This empirical case study investigates two secondary school science teachers' assessments of 15-16 years old students in three separate groups in the practical part of a Swedish national test in chemistry. Data are gathered using two video cameras and three pairs of spy camera glasses. The results show that individual and independent assessments are difficult due to the social interactions that take place and the physical sources of errors that occur in this type of setting.