Introduction
IN 1963 THE International Organization for Standardization (ISO) recommended a new set of zero reference levels for pure-tone audiometers. The concern and confusion which resulted has prompted a review of the concept of audiometer zero. This discussion will not be concerned with the validity of ISO values. Instead, it will present reasons for a basic physical reference zero rather than one based on statistical studies of audiometric data. This type of problem is seen to be fundamental to the expression of other clinical laboratory measurements, as well as hearing threshold levels.After studying many of the surveys on which the ISO physiologic standard is based and some that were not included, we agree that the ISO values are probably the best available estimate of "the hearing acuity of selected young individuals, tested by appropriate methods under perfect audiometric conditions." The questions we would raise are: "Is such a standard