Philip E. Ciddor and Reginald J. Hill published an analytical expression and a detailed procedure in 1999 on how to compute the group refractive index of air [Appl. Opt.38, 1663 (1999).APOPAI0003-693510.1364/AO.38.001663]. The International Association of Geodesy (IAG) has been referring to this procedure in its official recommendations for the compensation of the index of refraction in geodetic observations since then. Equation (B2) of the published procedure, however, contains a sign error. For proper application, this error must be corrected accordingly.