Mean values (filled circles) and 68% credible intervals (horizontal bars) obtained from posterior distributions of τ, derived from CMB power spectra measurements including: WMAP 2013 (Hinshaw et al. 2013), Planck PR1 + WMAP 2014 (Planck Collaboration XV 2014), Planck PR2 + WMAP 2016 (Planck Collaboration XIII 2016), Planck-LFI 2016 (Planck Collaboration XIII 2016), Planck-HFI 2016 (Planck Collaboration Int. XLVII 2016), WMAP+LFI 2018 (Weiland et al. 2018), Planck PR3 2020 (Planck Collaboration VI 2020), Planck 2020 (Pagano et al. 2020), WMAP+LFI 2022 (Paradiso et al. 2023), Planck PR4 2022 (Planck Collaboration Int. LVII 2020). The shaded grey region highlights the cosmic variance limit on the measurement of τ, centred on the value from Planck PR4 2024 (Tristram et al. 2024).