Many studies have concluded that the manual splitting of tablets, with and without tablet splitting devices, does not yield the desired results of an even split.
Peek (1) et al., (2002) studied tablet splitting by "elderly patients" aged 59-70.
They found that "tablet splitting resulted in an unacceptably high incidence of weight variation", and recommended that "standards should be developed to uniformity of split tablets."
"Accuracy of Tablet Splitting by Elderly Patients." The Journal of the American Medical Association 288 No.4 (2002):139-145.