Ratio of various error estimation procedures to the errors obtained with the data-based analytical estimate. At each frequency the numerator is given by: (i) the scatter within an ℓ bin in simulations (solid black line); (ii) the scatter within an ℓ bin in the data (solid dashed black line); (iii) the scatter of bins across simulated realizations (solid coloured line); (iv) the analytical errors calculated from the simulations (dashed coloured line); and (v) the scatter across realizations for the cross-correlation between the simulated temperature map and the lensing potential reconstructed from the data (coloured dot-dashed line). The grey envelope is the precision of the simulated errors expected from 100 simulations (shown as a spread around unity).