Test Early and Often
Frequent testing yields benefits for both you and your students. Early testing furnishes students with feedback they can use to optimize their course performance. Frequent testing gives students more opportunities for success, reducing the penalty for any single poor performance. By providing more testing opportunities the effects of random errors, such as student misinterpretation and distractions, tend to weaken. Consider giving a quiz early in the semester so that students have an idea of what the exams will be like.
Nilson, L.B (2010). Teaching At Its Best: A Research-Based Resource for College Instructors. San Francisco, CA: Jossey-Bass.