Here is answer from prof. Stig Arlinger, Linköping University:
Sources for data in ISO 389-1 (1998):
 WEISSLER, P.G. International Standard Reference Zero for Audiometers, J. Acoust. Soc. Amer., 44, 1968, pp. 264-275.
 COX, Jr., J.R. and BILGER, R.C. Suggestion Relative to the Standardization of Loudness-Balance Data for the Telephonics TDH-39 Earphone, J. Acoust. Soc. Amer., 32, 1960, pp. 1081-1082.
 WHITTLE, L.S. and DELANY, M.E. Equivalent Threshold Sound-Pressure Levels for the TDH-39/MX41-AR Earphone, J. Acoust. Soc. Amer., 39, 1966, pp. 1187-1188.
 MICHAEL, P.L. and BIENVENUE, G.R. A comparison of acoustical performance between a new one-piece earphone cushion and the conventional two-piece MX-41/AR cushion, J. Acoust. Soc. Amer., 67(2), 1980, pp. 693-698.
 ROBINSON, D.W. A proposal for Audiometric zero referred to the IEC artificial ear, UK National Physical Laboratory, Acoustics Report Ac 85 (1978).
 ARLINGER, S. Normal thresholds of hearing at preferred frequencies, Scand. Audiol., 11, 1982, pp. 285-286.
 RASMUSSEN, O. Reference equivalent threshold sound pressure levels for headphones at one-third octave standard frequencies, Internal Report No. 14, 1981, The Acoustics Laboratory, Technical University of Denmark, Lyngby.
 BRINKMANN, K. and RICHTER, U. Determination of the normal threshold of hearing by bone conduction using different types of bone vibrators, Audiological Acoustics, 22, 1983, pp. 62-85 and 114-122.
 BENWELL, D.A. and HUSSEY, R.G. Reference equivalent threshold sound pressure levels at 5 and 6.3 kHz using Telephonics TDH
Sources for data in ISO389-2 (1994):
 Wilber, L.A., Kruger, B. and Killion, M.C. Reference thresholds for the ER-3A insert earphone. J. Acoust. Soc. Amer., 83 (2), 1988, pp. 669-676. 2
 Arlinger, S. and Kinnefors, C. Reference equivalent threshold sound pressure levels for insert earphones. Scand. Audiol., 18, 1989,pp. 195-198.
 Brinkmann, K. and Richter, U. Reference zero for the calibration of pure tone audiometers equipped with insert earphones. Acustica, 70, 1990, pp. 202-207.
Since 2009 there is a standard that specifies conditions for obtaining reference data, ISO 389-9 “Acoustics – Reference zero for the calibration of audiometric equipment – Part 9: Preferred test conditions for the determination of reference hearing thresholds”.
I do not have access to ISO documents 389-1 or 389-2 which may provide the answers or the references needed to answer this question. I’m hoping one of you may know the answer or direct me to an appropriate reference.
Back in the early days (Beasley, 1935, Dadson and King, 1952 and Glorig, 1954, 1955) of developing norms for hearing threshold it was shown that norms varied based on whether the data were collected using “survey” methods (large N but less rigorous screening and less precise threshold measurement methods) or a “lab-based” (generally smaller N but more stringent exclusion and more precise method for threshold assessment) to derive threshold (See Glorig, 1956).
My question is: what method was used to derive the current norms? It would be most helpful if someone could provide the reference(s) for the studies used to determine audiometric zero in the current ISO standards.