This study sought to determine the effects of prior knowledge on the achievement of undergraduates in an economics course at the Open University of the Netherlands (OuN). A total the four law and seven economics students were given four instruments to test prior knowledge of economics, presented with a learning unit from the course "Economics and Money," and then post-tested on subject mastery. The results indicated that the second general measure of expertise, comprising the requisite test scores and the test scores for the subject-oriented expertise test, explained up to 42 percent of the variance in the posttest scores. Further analysis revealed that optimal requisite knowledge was an important component of student expertise. The study also found that the estimation of expertise level through self-assessment by the students was not very reliable. It is suggested that other dimensions, such as psychological and epistemological dimensions, can be put forward to analyze expertise. (Contains 20 references.) (MDM)