Re: Visual predictive check!

Luis, A very good suggestion. I agree this is an excellent piece of work dealing with the LLOQ problem. It provides a clear example of the benefits of the YLO option in NONMEM for dealing with censored data. By the way the first author is Wonkyung BYON -- not Byron. Best wishes, Nick [EMAIL PROTECTED] wrote: > Ken and All, > > The recent paper on JPP "Impact on censoring data below an arbitrary quantification limit on structural model misspecification" 2008, 35:101-16, by Byron, Fletcher and Brundage is still fully available on line and it speaks volumes about bioanalytical motivated LLOQ and pharmacokinetics modeling. Just for those who haven't read it, I vividly reccomend so. > > Cheers > > --------------------------------------------------------------- > Luis M. Pereira, Ph.D. > Assistant Professor, Pharmacometrics > Massachusetts College of Pharmacy and Health Sciences > > Childrens Hospital Boston / Harvard Medical School > 179 Longwood Ave, Boston, MA 02115 > Phone: (617) 732-2905 > Fax: (617) 732-2228 > > ------------------------------------------------------------------------ > *From:* [EMAIL PROTECTED] on behalf of Ken Kowalski > *Sent:* Fri 5/23/2008 11:22 AM > *To:* 'Nick Holford'; [email protected] > *Subject:* RE: [NMusers] Visual predictive check! > > Nick, > > Yes, I'm making the assumption that a measured concentration cannot be > negative. Educate me about chemical assays. Can you get troughs rather > > than peaks in a chromatogram such that the area below zero is integrated and > > reported as a negative concentration? If so, what would happen if you > > assayed a bunch of pre-dose samples (before drug is administered) where the > > true mean concentration is zero? Would we get measured concentrations > symmetrically distributed about zero (with about 50% of the measured > > concentrations reported as negative and 50% positive)? If so, then a normal residual error model may indeed be appropriate. > > Ken -- Nick Holford, Dept Pharmacology & Clinical Pharmacology University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New Zealand [EMAIL PROTECTED] tel:+64(9)373-7599x86730 fax:+64(9)373-7090 www.health.auckland.ac.nz/pharmacology/staff/nholford