 Early studies reported high success rates : in one study in 1925 , 60 % of patients became seizure @-@ free , and another 35 % of patients had a 50 % reduction in seizure frequency . These studies generally examined a cohort of patients recently treated by the physician ( what is known as a retrospective study ) and selected patients who had successfully maintained the dietary restrictions . However , these studies are difficult to compare to modern trials . One reason is that these older trials suffered from selection bias , as they excluded patients who were unable to start or maintain the diet and thereby selected from patients who would generate better results . In an attempt to control for this bias , modern study design prefers a prospective cohort ( the patients in the study are chosen before therapy begins ) in which the results are presented for all patients regardless of whether they started or completed the treatment ( known as intent @-@ to @-@ treat analysis ) . 
