Formative assessments are an important component of massive open online courses (MOOCs), online courses with open access and unlimited student participation. Accurate conclusions on students' proficiency via formative, however, face several challenges: (a) students are typically allowed to make several attempts; and (b) student performance might be affected by other variables, such as interest. Thus, neglecting the effects of attempts and interest in proficiency evaluation might result in biased conclusions. In this study, we try to solve this limitation and propose two extensions of the common psychometric model, the Rasch model, by including the effects of attempts and interest. We illustrate these extensions using real MOOC data and evaluate them using cross-validation. We found that (a) the effects of attempts and interest on the performance are positive on average but both vary among students; (b) a part of the variance in proficiency parameters is due to variation between students in the effect of interest; and (c) the overall accuracy of prediction of student's item responses using the extensions is 4.3% higher than using the Rasch model.