We examine a flexible assessment system that allows students to determine the weights allocated to each course component and to re-allocate the weights in response to achieved scores. The flexibility is intended to encourage students' participation in the learning process, thereby promoting self-regulated learning skills. We compare this assessment system to a traditional system and to another system, in which the students may choose their own weights but cannot change them later in the term. We examine the impact on students' motivation as measured by the Motivated Strategies for Learning Questionnaire (MSLQ), as well as the impact on grades and students' attitudes. Our results indicate little improvement in motivation, grades or attitudes when students design and commit to their grading plans early in the term. However, the more flexible system, which allows for re-allocation, appears to improve students' grades, self-reported motivation and attitudes.