Values of τ measured on data using different sky masks. The red points show the median values of the τ distributions measured on the masks shown in Fig. 7. The blue point represents the value of τ obtained using the 50% alternative mask shown in the middle part of Fig. 7. Intense and fainter bars show 68% and 95% confidence levels, respectively. The vertical line shows the best-fit value from the default analysis, i.e., the 50% mask.