These data are from the calibration of an atomic absorption
spectrometer. The Standards run from a blank sample
(assumed to have value 0) to 50.  Absorbance is measured three
times for each standard.  (It is NOT that three samples are run, 
it is that each sample has its value recorded three times.)
Column X1 shows the values recorded before running a series of
unknown samples. Column X2 shows the values recorded after running
a series of unknown samples.  The difference between X1 and X2 is
a measure of instrument drift over time.

Data source:  Dr. John Adgate, University of Minnesota School of
Public Health