Re: Concentration values below assay limits

From lewis@c255.ucsf.edu Tue May 20 16:32:53 1997 Subject: Re: Concentration values below assay limits I have the feeling this has been discussed before... Briefly, BQL values can convey information, especially if they occur rather isolated in time (that is, there are no above QL levels "nearby"), and should then be included in the data analysis. While the very best way to do so is not yet known, here is a way that works pretty well: 1. Use an error model that has an additive and proportional component; e.g., Y = F + F*EPS(1) + EPS(2) 2. Fix var(eps(2)) to .25*QL**2. Say QL is .5 mcg/ml, then use, e.g., $SIGMA .04 .0625 FIX 3. Record BQL values as QL/2; i.e., for the above example as .25.