Re: Concentration values below assay limits

From lewis@c255.ucsf.edu Tue May 20 17:11:08 1997 Subject: Re: Concentration values below assay limits Basically, yes, although the formulas are actually: 1. var(eps(2)) = (f*QL)**2, where f can be taklen to be .5, but see below. 2. Record BQL values as QL/2 The rationale is 1. the CV at QL is about f*100% (many define QL to be at f=.2; f=.5 gives BQL observations even less weight and is conservative in that sense). 2. If all we know is that an observation is BQL, a reasonable ignorance assumption is that the true value is drawn from a uniform distribution with support on 0 - QL; a reasonable imputed value is then the mean of this distribution, viz., QL/2. -LBS.