We demonstrate that there are systematic scale errors in the [ Fe/H ] values determined by the Hamburg/ESO Survey ( and by inference by the HK Survey in the past ) for certain extremely metal poor highly C-enhanced giants .
The consequences of these scale errors are that a ) the fraction of carbon stars at extremely low metallicities has been overestimated in several papers in the recent literature b ) the number of extremely metal poor stars known is somewhat lower than has been quoted in the recent literature c ) the yield for extremely metal poor stars by the HES Survey is somewhat lower than is stated in the recent literature .
A preliminary estimate for the frequency of Carbon stars among the giants in the HES sample with -4 < [ Fe/H ] < -2.0 dex is 7.4 \pm 2.9 % ; adding an estimate for the C-enhanced giants with [ C/Fe ] > 1.0 dex without detectable C _ { 2 } bands raises the fraction to 14 \pm 4 % .

We rely on the results of an extensive set of homogeneous detailed abundance analyses of stars expected to have [ Fe/H ] \leq - 3.0 dex selected from the HES to establish these claims .
We have found that the Fe-metallicity of the cooler ( T _ { eff } \lesssim 5200 K ) C-stars as derived from spectra taken with HIRES at Keck are a factor of \sim 10 higher than those obtained via the algorithm used by the HES project to analyze the moderate resolution follow-up spectra , which is identical to that used until very recently by the HK Survey .
This error in Fe-abundance estimate for C-stars arises from a lowering of the emitted flux in the continuum bandpasses of the KP ( 3933 Å line of Ca II ) and particularly the HP2 ( H \delta ) indices used to estimate [ Fe/H ] due to absorption from strong molecular bands .