On the limitations of statistical absorption studies with the Sloan
  Digital Sky Surveys I--III
  We investigate the limitations of statistical absorption measurements with
the SDSS optical spectroscopic surveys. We show that changes in the data
reduction strategy throughout different data releases have led to a better
accuracy at long wavelengths, in particular for sky line subtraction, but a
degradation at short wavelengths with the emergence of systematic spectral
features with an amplitude of about one percent. We show that these features
originate from inaccuracy in the fitting of modeled F-star spectra used for
flux calibration. The best-fit models for those stars are found to
systematically over-estimate the strength of metal lines and under-estimate
that of Lithium. We also identify the existence of artifacts due to masking and
interpolation procedures at the wavelengths of the hydrogen Balmer series
leading to the existence of artificial Balmer $\alpha$ absorption in all SDSS
optical spectra. All these effects occur in the rest-frame of the standard
stars and therefore present Galactic longitude variations due to the rotation
of the Galaxy. We demonstrate that the detection of certain weak absorption
lines reported in the literature are solely due to calibration effects.
Finally, we discuss new strategies to mitigate these issues.
