  Cosmology contributes a good deal to the investigation of variation of
fundamental physical constants. High resolution data is available and allows
for detailed analysis over cosmological distances and a multitude of methods
were developed. The raised demand for precision requires a deep understanding
of the limiting errors involved. The achievable accuracy is under debate and
current observing proposals max out the capabilities of todays technology. The
question for self-consistency in data analysis and effective techniques to
handle unknown systematic errors is of increasing importance. An analysis based
on independent data sets is put forward and alternative approaches for some of
the steps involved are introduced.
