Re: time variant covariate ?

Date: Thu, 10 Jun 1999 10:48:53 -0700 From: LSheiner <lewis@c255.ucsf.edu> Subject: Re: time variant covariate ? Time-varying covariates can be handled. The easiest way is to approximate them as a step function as follows: A covariate is considered to be constant at the value specified on a given record, over the time period from the previous record to the current one. Thus, if covariate X has value 0 at time 0 and value 10 at time 10, one might add "other" records at times 2,4,6,8 with recorded values of X = 1,3,5,7, with values 0 and 10 at times 0 and 10, respectively. A PK parameter, e.g., clearance, which is a function of X, for example TVCL = THETA(1) + THETA(2)*X will then change in steps; TVCL will have the value THETA(1) + THETA(2) for the advance from time 0 to time 2; THETA(1) + 3* THETA(2) for the advance from time 2 to time 4, etc. If the covariate must change continuously, this too can be handled, but only by using differential equations and defining the function that yields X(T) in the $DES block. LBS. Lewis B Sheiner, MD Professor: Lab. Med., Biopharm. Sci., Med. Box 0626 voice: 415 476 1965 UCSF, SF, CA fax: 415 476 2796 94143-0626 email: lewis@c255.ucsf.edu