For a popPK model I am working on, I estimated interoccasion variability (IOV) for CL for 4 different occasions, assuming the IOV estimates to be the same for all 4 occasions due to limited data availability. Therefore, I obtained 4 identical omega estimates and 4 identical standard error estimates for the four occasions (i.e. IOV on CL for occasion 1 = IOV on CL for occasion 2 = IOV on CL for occasion 3 = IOV on CL for occasion 4). My understanding is that I only need to report one estimate and its standard error for IOV on CL as my model output, instead of reporting the same set of values 4 times defined by the occasions. However, I do not know what to report for the shrinkage associated with IOV on CL. In the Omega sheet, 4 different shrinkage values were shown associated with the 4 occasions, ranging from 0.3 to 0.9. From looking at published studies, people report only one shrinkage value for each IOV tested. How can I obtain an overall shrinkage value to report for the IOV on CL? The number of data points for each occasion are different (e.g. intense sampling on occasion 1 and 4, sparse sampling on occasion 2 and 3), in case this matters.
Thank you smoukasassi1 for your reply! Yes, I do agree with you. I understand why there are separate shrinkage values for each occasion. Therefore, I am very confused by what I’ve seen in popPK literature because when IOV shrinkage is reported, only 1 value is usually presented, such as in the example below (Chan et al., Pharm Res. 2020 Jan 6;37(2):25.
doi: 10.1007/s11095-019-2752-y.).
For IOV to make sense, there have to be multiple occasions defined. Therefore, several shrinkage values should have been estimated for each occasion. I should probably rephrase my question:
Why do people report only one shrinkage value for each IOV? What is this single shrinkage value telling us and is this standard practice?
Is there a way to obtain a single shrinkage value?
Or is this due to some algorithm differences in how IOV is calculated between NONMEM and Phoenix NLME?
