Recombining Six Lensed Quasars: Where the Time-Delay Hubble Constant Error Budget Actually Sits

The six lensed quasars of the H0LiCOW sample are recombined from their published individual Hubble constant inferences, and the combined error budget is decomposed into statistical and systematic parts. Inverse-variance weighting of the six published values gives H0 = 73.42 km/s/Mpc with a purely statistical uncertainty of 1.69 km/s/Mpc. Adding the three systematic floors adopted here (Fermat-potential modelling at 2.0 per cent, external convergence at 1.5 per cent and residual line-of-sight structure at 1.0 per cent, a combined 2.69 per cent) raises the total uncertainty to 2.6 km/s/Mpc, or 3.54 per cent. The six individual values span 12.3 km/s/Mpc and are mutually consistent, with chi-squared 4.86 for five degrees of freedom. The systematic floors, not the photon statistics of the existing sample, set the precision; doubling the sample without lowering the floors would improve the total by less than a fifth. This study is a recomputation from published values, simulated and not measured.