  We propose a forward-backward multiplicity correlation function $C^N_{FB}$,
which is experimentally accessible, to measure the noncollectivity
contribution. We find that $C^N_{FB}$ is sensitive to the jet contribution for
the particle-rich case. Surprisingly, it will automatically decrease for the
particle-rare case. Our study indicates that similar decreasing trend observed
previously is mainly driven by particle scarcity instead of jets. The function
is studied in Au+Au collision at $\sqrt{s_{NN}}=200$ GeV with a multiphase
transport model (AMPT). We find that the jet fraction is about 10% at
transverse momentum ($p_T$) around 2.5 GeV/$c$ and reaches up to 30% at 3.5
GeV/$c$. The implication of this study in the investigation of the
noncollectivity contribution in elliptic anisotropy parameter $v_2$ is also
discussed.
