RE: CV for exponential model

From:Matthew Riggs Subject:RE: [NMusers] CV for exponential model Date:Wed, 5 Feb 2003 06:19:36 -0800 (PST) Luciane, As Bill stated, the CV = sqrt(omega(n))*100 for a proportional structure: V= TVV*(1+ETA(n)) and for an additive structure (V= TVV+ETA(n)), the CV = sqrt(omega(n))/theta(n)* 100. And as Yaning stated, the FO method applies a first order Taylor expansion about an assumed mean eta of zero; mathematically collapsing the exponential error form into a proportional form = same calculation for CV. However, it's been my impression that when using FOCE or LAPLACIAN with an exponential structure, the math does not "collapse" so easily and you need to consider calculating the CV% = 100*sqrt(exp(omega(n))-1), particularly with "large" values of omega (e.g., >=0.16). It's easy enough to see the deviation you'd get by using the two methods (proportional vs. exponential), especially at higher omegas, by comparing the CV% values across a range of omegas in a spreadsheet. This was addressed back in 1997 by Stuart Beal, see: http://www.cognigencorp.com/nonmem/nm/98sep261997.html -Matt _________________________________________