Fisher, R.A. (1925). Statistical Methods for Research Workers. Oliver and Boyd (Edinburgh). ISBN 0-05-002170-2.
Fisher, R. A (1948). "Questions and answers #14". The American Statistician. 2: 30<U+2013>31. doi:10.2307/2681650. JSTOR 2681650.
See also Stouffer's method for combining Z scores, which allows weighting.
Stouffer, S. A. and Suchman, E. A. and DeVinney, L. C. and Star, S. A. and Williams, R. M. Jr. (1949) The American Soldier, Vol. 1 - Adjustment during Army Life. Princeton, Princeton
University Press.