Investigating the Use of Mastery-Style Online Homework Exercises in   Introductory Algebra-based Mechanics in a Controlled Clinical Study
  Homework in introductory physics represents an important part of a student's learning experience; therefore choosing the manner in which homework is presented merits investigation. We performed three rounds of clinical trials comparing the effects of mastery-style homework vs traditional-style homework with students in both algebra-based and calculus-based introductory mechanics. Results indicate a benefit from mastery-style over traditional-style homework, principally for weaker students who are less familiar with the material being covered and on questions that are nearer transfer to the study materials. 
William R. Evans, Mats A. Selen
physics.ed-ph