  Linear spectropolarimetry is a ``photon-hungry'' observing technique,
requiring a specific sequence of observations to determine the Stokes Q and U
parameters. For dual-beam spectropolarimeters, the parameters can be ideally
determined using observations at N=2 retarder plate positions. The additional
polarization signal introduced by instrumental effects requires the redundancy
of N=4 observations to correct for these effects. We wish to determine if the
``instrumental signature corrections'', eQ and eU, are identical for
observations with dual-beam spectropolarimeters. For instances when
observations were acquired at N=3 retarder plate angles, we wish to determine
if the complete measurement of one Stokes parameter and the associated
instrumental signature correction can be used to determine the other Stokes
parameter. We constructed analytical and Monte Carlo models of a general
dual-beam spectropolarimeter to study the factors affecting the assumption
eQ=eU. We compared these models with VLT FORS1 spectropolarimetry. We find that
eQ-eU=0, with the variance around zero (\Delta(eQ-eU)) being directly related
to the signal-to-noise ratio of the observations. Observations of a polarized
standard star, over the period 2002-2007, show that the assumption of eQ-eU=0
is generally true, although the absolute values of eQ and eU vary between
observational epochs. Incomplete spectropolarimetry datasets can be analyzed
under the assumption that eQ=eU. This property of the analysis of
spectropolarimetry, with dual beam spectropolarimeters, can be used to assess
the quality of the reduction for complete data.
