File 1 is from an ideal supercapacitor and displays a linear GCD profile, as expected for a device where the charge is solely stored by non-Faradaic ion adsorption in the electric double-layer. As shown in Figure , most participating groups obtained comparable results when calculating the total capacitance from File 1, with over 50% of participants lying within one sigma of the adjusted mean average. This is despite differences in analysis methods such as determining the discharge slope and the formulae used. See the Supplementary Information for detailed information on the analysis methods used by each participating group. However, extreme anomalies were observed, accounting for approximately 14% of reported results. These anomalies were due to incorrect formulae used to calculate the total capacitance, with the results being either too small or too large by a factor of two (SI Section 3).