The purpose of this part of IEC 60118 is to describe methods for a test which simulates the acoustical effects of a median adult wearer on the performance of a hearing aid. It establishes certain guidelines for simulated in situ measurements of hearing aids; it describes a simplified method for simulated in situ measurements of hearing aids and a description for determination of the directivity index (DI) of directional microphones in hearing aids in the horizontal plane. In addition this second edition now specifies tolerances. Conformance to the specifications in this International Standard is demonstrated only when the result of a measurement, extended by the actual expanded uncertainty of measurement of the testing laboratory, lies fully within the tolerances specified in this International Standard extended by the values for Umax