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Hi all, I'm faced with an analysis of data concerning chemometrics, which I'm not very familiar with. It seems to me that the experiment is quite poorly designed, so I'll try to explain what my brother (actually the chemical engeneer) is looking for. Say a certain substance contains No2 and No3 and we are interested in assessing the concentration of No3 by spectroscopy. We know that absorbance readings for No2 are ok, but those for No3 are somehow influenced by No2 concentrations. So first the instrument is calibrated for No2 and No2 concentration is estimated. Secondly No2 is transformed in No3 by some obsidation process, the instrument is independently calibrated for No3 and the total concentration of No3 is estimated, assuming it is given by the sum of the initial concentration of No3 plus that due to having transformed No2 in No3. Finally the initial concentration of No3 is estimated by subtracting the formerly estimated No2 concentration from the total No3 concentration. Several replications of this experiment are available together with the true concentrations of the substances, and my brother is obviously interested in assessing the quality of his No3 concentration estimation procedure. My first impression is that unless the two calibrations are performed in a dependent way there is no way to use these data. What my brother is trying to do is essentially using two independent estimators for assessing two strongly dependent quantities (total No3 and initial No2). Given that the obsidation mechanism can't be explicitly modeled because of the presence of interactions within the substance at hand, does anybody have any idea on how to use these data for the purpose? My suggestion for a future experiment taking into account the interaction between the two estimators was simply to perform the two calibrations on the same No2 based substance, before and after obsidation. Does that sound reasonable? What could be an alternative strategy? Alessio Alessio Pollice Dipartimento di Scienze Statistiche Università degli Studi di Bari Italy %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%