We carry out a systematic search for extremely metal poor (XMP)
galaxies in the spectroscopic sample of Sloan Digital Sky Survey (SDSS)
data release 7 (DR7). The XMP candidates are found by classifying all the
galaxies according to the form of their spectra in a region 80A wide
around H\alpha. Due to the data size, the method requires an
automatic classification algorithm. We use k-means.
Our systematic search renders 32 galaxies having negligible [NII] lines,
as expected in XMP galaxy spectra. Only nine of them have been previously
identified as XMP galaxies in the literature -- the rest are new.
XMP galaxies are rare; they represent 0.01% of the observed galaxies.
Only 82 were known before, and we add 23 new targets.
Although the final metallicity estimate of all candidates
remains pending, strong-line empirical calibrations indicate
a metallicity smaller than one-tenth solar, with the
oxygen metallicity of the nine known targets
being 12+\log(O/H)~7.48+- 0.19.
Since the SDSS catalog is limitted in apparent magnitude,
we have been able to estimate the volume number density of XMP
galaxies in the local universe, which turns out to be
(1.32+-0.23) x 10^{-4}~ Mpc^{-3}. The XMP galaxies
constitute 0.1\%\ of the galaxies in the local volume,
or ~ 0.2% considering only emission line galaxies.
All but four of our candidates are blue compact dwarf galaxies
(BCDs), and 24 of them have either cometary shape or are formed by
chainned knots.