  We estimate the number of quark jets in QCD multi-jet final states at hadron
colliders. In the estimation, we develop the calculation of jet rates into that
of quark jet rates. From the calculation, we estimate the improvement on the
signal-to-background ratio for a signal semi-analytically by applying
quark/gluon discrimination, where the signal predicts many quark jets. We
introduce a variable related to jet flavors in multi-jet final states and
propose a data-driven method using the variable to reduce systematic
uncertainties of analysis results. As the same with the semi-analytical result,
the improvements on the signal-to-background ratio using the variable in
Monte-Carlo analysis are estimated.
