Threats to construct validity should be reduced to a minimum. If true, sources of bias, namely raters, items, tests as well as gender, age, race, language background, culture, and socio-economic status need to be spotted and removed. This study investigates raters' experience, language background, and the choice of essay prompt as potential sources of biases. Eight raters, four native English speakers and four Persian L1 speakers of English as a Foreign Language (EFL), scored 40 essays on one general and one field-specific topic. The raters assessed these essays based on Test of English as a Foreign Language (TOEFL) holistic and International English Language Testing System (IELTS) analytic band scores. Multifaceted Rasch Measurement (MFRM) was run to find extant biases. In spite of not finding statistically significant biases, several interesting results emerged illustrating the influence of construct-irrelevant factors such as raters' experience, L1, and educational background. Further research is warranted to investigate these factors as potential sources of rater bias.