An Empirical Study of the Effect of Granting Multiple Tries for Online   Homework
  When deploying online homework in physics courses, an important consideration is how many attempts should be allowed in order to solve numerical free-response problems. While on the one hand, the maximum number of attempts should be large enough to allow learners mastery of the concepts and disincentivize copying, on the other hand, too many allowed attempts encourage counter-productive problem-solving behavior. In a study of an introductory calculus-based physics course for scientists and engineers which had different numbers of allowed attempts in different semesters, it was found that except for the initial tries, students are not making productive use of later attempts, and that in fact higher numbers of allowed attempts lead to less successful homework completion. 
Gerd Kortemeyer
physics.ed-ph