Sum of the cross sections of positive and negative charge muons from W boson decays measured in p-Pb collisions at $\snn = 5.02$ TeV in the rapidity region $-4.46 < y_{\rm cms} < -2.96$ (left panel) and $2.03 < y_{\rm cms} < 3.53$ (right panel) as a function of centrality. The cross sections are normalised by the number of binary collisions $\langle N_{\rm coll}^{\rm mult} \rangle$. The vertical bars (open boxes) represent the statistical (systematic) uncertainties. The correlated global uncertainties include the MB cross section, normalisation, $A \times \epsilon$ corrections and tracking and trigger systematics. A dotted line is drawn at the value of the centrality-integrated cross section to guide the eye.