The exact bias of the Banzhaf measure of power when votes are neither equiprobable nor independent

This paper discusses a numerical scheme for computing the Banzhaf swing probability when votes are neither equiprobable nor independent. Examples indicate a substantial bias in the Banzhaf measure of voting power if neither assumption is met. The analytical part derives the exact magnitude of the bias due to the common probability of an affirmative vote deviating from one half and due to common correlation in unweighted simple-majority games. The former bias is polynomial, the latter is linear. A modified square-root rule for two-tier voting systems that takes into account both the homogeneity and the size of constituencies is also provided.