I often use subjective assessment for my online students via written essay type answers. A study showed the value of subjective assessment used in anesthesiology residency e-leanrning ( Chu et. al, 2013).
Reference
Chu LF, Ngai LK, Young CA, Pearl RG, Macario A, Harrison TK. (2013). Preparing Interns for Anesthesiology Residency Training: Development and Assessment of the Successful Transition to Anesthesia Residency Training (START) E-Learning Curriculum. J Grad Med Educ. ;5(1):125-9. doi: 10.4300/JGME-D-12-00121.1.