Random effect on ALAG between dose events

Dear NMusers, I'm modeling a PK data set with a discrepancy between the documented dosing time and the actual dosing time. According to our clinical practice, actual dosing time is always >= documented time. I added a ALAG with IIV to address this issue using the following formulation. ALAG1 = THETA(5) * EXP(ETA(5)) This indeed improved the model fitting quite a lot. However, this parameterization does not reflect the reality as I expect the ETAs should vary between each dosing event rather than only between patients. So I expect a "inter dose event variability" would better make sense to this end. Since there are too many dosing events per patients, a IOV-like approach is doable but not preferred. And it may not accurately reflect "inter dose event variability" either. I was wondering if there is any good solution to this problem? Any comments are very much appreciated! Warm regards, Tingjie Guo