Editor—The body weight of the preterm babies in the study by Tang et al predicted the total body water better than any other single anthropomorphic measure, and that it accounted for 99% of the variations seen in body water measurements.1 The authors then used a variety of other measurements and anthropomorphic characteristics and combined them in a number of equations and picked the best (involving a constant, body weight, body weight squared, and the length of the foot squared divided by electrical impedance taken under very specific conditions). They claim …