Among the screening candidates identified to be on the convex hull, 3,938 n-type materials show their standard deviation of effective mass less than one, which indicates that most stable materials are electronically isotropic, as is perhaps to be expected. By comparison however, the maximum standard deviation of effective mass for n-type materials reaches an outlandish 1,910,154. Such a huge standard deviation as this is meaningless and arises as an artifact from the effective mass. Because the derivative of a perfectly flat band is infinite, there is no upper limit to the value that can be presented by the standard deviation. What exactly is a high enough effective mass for the band to be considered flat is entirely arbitrary and so a heuristic approach is still needed. As examples for comparison, we include experimentally reported values of effective mass in Table . We include more details on the effective mass at a, b and c crystallographic directions for the selected 1,000 p-type and 1,000 n-type materials in the supplementary materials (see Figures and ).