$< N_{\rm {ch}}>\nu^{corr}_{(+-,dyn)}$ (left axis) and $D$ (right axis) as a function of $\Delta\eta$ window for three different centrality bins in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV. The data points are fitted with the functional form, erf($\Delta\eta/\sqrt{8}\sigma_{\rm f}$). The dashed lines correspond to the extrapolation of the fitted curves. The points are shifted minimally along $x$-axis for clear view Both statistical (error bar) and systematic (box) errors are shown.