Purity of the isolated-photon sample as a function of \ptg\ calculated using Eq. \eqref{eq:ABCDpurityMC} and with the statistical and systematic uncertainty discussed in Sect. \ref{sec:sys_unc} for $R=0.2$ and $0.4$. The curves (plain for low \ptg, dashed for high \ptg) are the two sigmoid functions as defined in Eq. \eqref{eq:sigmoid}, obtained by fitting the points as explained in the text.