  The extended Baryon Oscillation Spectroscopic Survey (eBOSS) is one of the
first of a new generation of galaxy redshift surveys that will cover a large
range in redshift with sufficient resolution to measure the baryon acoustic
oscillations (BAO) signal. For surveys covering a large redshift range we can
no longer ignore cosmological evolution, meaning that either the redshift
shells analysed have to be significantly narrower than the survey, or we have
to allow for the averaging over evolving quantities. Both of these have the
potential to remove signal: analysing small volumes increases the size of the
Fourier window function, reducing the large-scale information, while averaging
over evolving quantities can, if not performed carefully, remove differential
information. It will be important to measure cosmological evolution from these
surveys to explore and discriminate between models. We apply a method to
optimally extract this differential information to mock catalogues designed to
mimic the eBOSS quasar sample. By applying a set of weights to extract redshift
space distortion measurements as a function of redshift, we demonstrate an
analysis that does not invoke the problems discussed above. We show that our
estimator gives unbiased constraints.
