Histograms of the enclosed dark matter mass within R_HI for the ALFALFA sample and the TNG100 sample in five mass bins. The measurements for the ALFALFA sample are shown in blue, the true values of the TNG100 sample (TNG100-true) are plotted in orange, and the results obtained from the mock observations of TNG100 sample (TNG100-mock) are shown in red. Median values for each dataset are indicated by a vertical dashed line matching the colour of the sample.