Online-quizzes are an economic and objective method for formative assessment in universities. However, closed questions have been criticized for promoting shallow learning and resulting often in poor learning outcomes. These disadvantages can be overcome by embedding closed questions in effective instructional designs involving feedback. In the present field study, a final sample of N = 496 students completed the same online quiz, consisting of 60 true-false statements on the biological bases of psychology in two sessions. In order to enhance the benefit of formative testing on students' test achievement in Session 2, students received elaborate feedback (i.e., by providing explanations for the in-/correctness) for half of their answers in Session 1, and corrective feedback (i.e., just indicating the in-/correctness) for the other half. The results showed that students scored higher in Session 2 if elaborate feedback had been provided in Session 1, compared with when corrective feedback was provided. More specifically, students profited more from elaborate feedback on incorrect answers in Session 1 than from feedback on correct answers. As a practical recommendation, self-administered formative tests with closed-question format should at least provide explanations why students' answers are incorrect.