We introduce a new grading system, the Cloud Teaching Assistant System (CTAS), as an additional element to instructor grading, peer grading and automated validation in massive open online courses (MOOCs). The grading distributions of the different approaches are compared in an experiment consisting of 476 exam participants. 25 submissions were graded by all four methods. 451 submissions were graded only by peer grading and automated validation. The results of the experiment suggest that both CTAS and peer grading do not simulate instructor grading (Pearson's correlations: 0.36, 0.39). If the CTAS and not the instructor is assumed to deliver accurate grading, peer grading is concluded to be a valid grading method (Pearson's correlation: 0.76).