RE: covariates

From: "Leonid Gibiansky" leonidg@metrumrg.com Subject: RE: [NMusers] covariates Date: Thu, September 16, 2004 8:46 am Nick, Renee, Allometric model may be a valid guess, but I would be cautious to take those laws for granted. You may test them, but at least the results should be checked: plots of random effects (computed by the allometric-scale model) against WT should be reviewed. If the plots reveal any WT-trends, then allometric scaling is not accounting for the entire WT-dependence. You may argue that the remaining dependences should be treated via gender-dependence (gender is highly correlated with WT) or age-dependence (for pediatric patients age and WT are very strongly correlated) but in any case, this should be considered. On the other hand, if the model without WT describes data better than the one with WT (you can check it via OF or just looking on the fit) then it make no sense to use allometric scaling in the model. I think it is better to report the results for pediatric patients (say, 0-5 year old) using base WT that is close to the mean/median WT in the target population rather than using base WT=70. At least, the parameter values for a typical patient in the target population should be provided. Screening of covariates can be done formally, via likelihood test (OF) or informally, just looking on the plots of random effects (computed from the base model) versus covariates. You should be able to catch all strong dependencies just by eye. Leonid ========================== Metrum Research Group LLC 15 Ensign Drive Avon, CT 06001 www.metrumrg.com