Sloan Digital Sky Survey ( SDSS ) repeat spectroscopic observations have resulted in multiple-epoch spectroscopy for \sim 2500 quasars observed more than 50 days apart .
From this sample , calibrating against stars observed simultaneously , we identify 315 quasars that have varied significantly between observations ( with respect to assumed non-variable stars observed concurrently ) .
These variable quasars range in redshift from 0.5 to 4.72 .
This is the first large quasar sample studied spectroscopically for variability and represents a potentially useful sample for future high-redshift reverberation mapping studies .
This also marks the first time the precise wavelength dependence of quasar variability has been determined , allowing both the continuum and emission line variability to be studied .
We create an ensemble difference spectrum ( bright phase minus faint phase ) covering rest-frame wavelengths from 1000Å to 6000Å .
This average difference spectrum is bluer than the average single-epoch quasar spectrum ; a power-law fit to the difference spectrum yields a spectral index \alpha _ { \lambda } = -2.00 , compared to an index of \alpha _ { \lambda } = -1.35 for the single-epoch spectrum .
This confirms that quasar continua are bluer when brighter .
The difference spectrum also exhibits very weak or absent emission line features ; the strongest emission lines vary only 30 \% as much as the continuum .
This small emission line variability with respect to the continuum is consistent with the Intrinsic Baldwin Effect .
Due to the lack of variability of the lines , measured photometric color is not always bluer in brighter phases , but depends on redshift and the filters used .
Lastly , the difference spectrum is bluer than the ensemble quasar spectrum only for \lambda _ { rest } < 2500 Å , indicating that the variability can not result from a simple scaling of the average quasar spectrum .