  Identification of jets originating from b quarks (b-tagging) is a key element
of many physics analyses at the LHC. Various algorithms for b-tagging have been
developed by the CMS experiment to identify b-tagged jets with a typical
efficiency between 40% and 70% while keeping the rate of misidentified light
quark jets between 0.1% and 10%. An important step, in order to be able to use
these tools in physics analysis, is the determination of the efficiency for
tagging b-jets. Several methods to measure the efficiencies of the lifetime
based b-tagging algorithms are presented. Events that have jets with muons are
used to enrich a jet sample in heavy flavor content. The efficiency measurement
relies on the transverse momentum of the muon relative to the jet axis or on
solving a system of equations which incorporate two uncorrelated taggers.
Another approach uses the number of b-tagged jets in top pair events to
estimate the efficiency. The results obtained in 2010 data and the
uncertainties obtained with the different techniques are reported. The rate of
misidentified light quarks have been measured using the "negative" tagging
technique.
