Abundances in extragalactic HII regions

This sample constitutes the largest and most homogeneous high-quality data set in existence for the determination of Yp. For comparison, and to improve the statistics in our investigation of systematic effects affecting the Yp determination, we have also considered a sample of 271 low-metallicity HII regions selected from Data Release 5 of the Sloan Digital Sky Survey.

Although this larger sample shows more scatter, it gives results that are consistent at the 2{sigma} level with our original sample. We have considered known systematic effects that may affect the ^4^He abundance determination.