Variable confounds with residual

[#1]

Hi,

l am attempting to analyse data for an interlab study with a mixed model, its a nested design with 5 variables, the dependant variable is titers, the structure of my analysis is as follows

Fixed Effect

Sample (30 of)

Random Effects

Sample*Lab (3 levels)
Sample*Day (3 Levels)
Sample*Run (2 Levels)
Replicate[Sample,Lab,Day,Run] (replicate has 2 levels)
Sample*Lab*Day

l get the error message that Replicate[Sample,Lab,Day,Run] confounds with residual, this is the first time i have encountered this message and i am not sure what its telling me, has anyone encountered this before and how did they overcome it??

Thanks

Mick

[#2]

Sample as you define and use it is a FIXED effect on the response.

Again, include the main effects to address the primary questions. You may include cross-terms to estimate and test interaction effects if you suspect that the effect of the other factors change from one sample to another. Is that a reasonable expectation? Is there precedent for it? Or can it be ruled out a priori?