Combined yield of muons from W$^-$ and W$^+$ decays with $\pt^\mu >$ 10 GeV/$c$, normalised by the average number of binary nucleon--nucleon collisions $\langle N_{\rm coll}^{\rm mult} \rangle$ in p-Pb collisions at $\sqrt{s_{\rm NN}}=8.16$ TeV in the Pb-going (left) and p-going (right) configurations. The bars and boxes correspond to statistical and systematic uncertainties respectively. The horizontal dashed line indicates the central value of the yield normalised to $\langle N_{\rm coll}^{\rm mult} \rangle$ measured for the 0--100\% centrality interval.