The combined LEP and SLD measurements of R b are about three standard deviations above the standard model. The measurement techniques employed are claimed to be highly reliable and systematically safe. R c , which is measured using different and inherently less reliable techniques, is found to be 1.7 standard deviations low. In this paper, I will examine what constraints the measurements of Ī±sā put on these measurements. I will also explore possible sources of systematics related to the models used to account for QCD effects in the processes measured
