Dear Manu, NONMEM doesn't care about negative values when analysing data in the log domain. The sure-fire test is to do the analysis both after log-transforming in the original units, and after multiplying by 1000 as Dennis suggests, and the results should be identical (if you adjust your afflicted starting values accordingly of course).
Kind regards, Rik Rik Schoemaker, PhD Occams Malandolaan 10 1187 HE Amstelveen The Netherlands www.occams.eu<http://www.occams.eu/> +31 20 441 6410 rik.schoema...@occams.eu<mailto:rik.schoema...@occams.eu> [cid:image003.png@01D1E8AE.09333740] From: owner-nmus...@globomaxnm.com [mailto:owner-nmus...@globomaxnm.com] On Behalf Of Dennis Fisher Sent: 27 July 2016 23:26 To: Bharadwaj, Manushree <manu...@ostatemail.okstate.edu> Cc: nmusers@globomaxnm.com Subject: Re: [NMusers] Log transformation of imputed BQL one thing that you could do is multiply all the values by 1000 (i.e., change the units) Dennis Fisher MD P < (The "P Less Than" Company) Phone / Fax: 1-866-PLessThan (1-866-753-7784) www.PLessThan.com<http://www.plessthan.com/> On Jul 27, 2016, at 11:11 AM, Bharadwaj, Manushree <manu...@ostatemail.okstate.edu<mailto:manu...@ostatemail.okstate.edu>> wrote: Dear friends, I log transformed the plasma concentrations in my data and I am using this log transformed data as my dependent variable (DV). I imputed the BQLs to LLOQ/2, which is my case is (1/2=0.5). The natural log of 0.5 is negative which mean that I have negative values in my DV column. While the parameter estimates look somewhat reliable, I have negative points plotted in the diagnostic plots such as DV vs PRED, DV and PRED vs TIME, etc. My questions are- 1. What is the best way of dealing with <1 DV/plasma concentration values while transforming into natural logarithm? 2. In my situation discussed above, where I have negative DV values in my data, has the NONMEM ignored or considered the negative numbers when estimating the parameter values? Thank you, Manu -- With regards- Manushree Bharadwaj, BVSc, PhD Candidate Graduate Teaching Associate Department of Physiological Sciences Center for Veterinary Health Sciences Oklahoma State University Stillwater, OK-74074
