Because of the extreme variety of conditions under which a hearing aid may operate when in use, it is desirable to determine the character and ranges of magnitude of differences between the performance of an instrument when worn and its performance as measured by the conventional laboratory procedures. The principal differences are due to the following factors: (1) differences between the 2-cc coupler and human ears, (2) the snugness of fit of the molded earpiece, (3) the size and shape of the acoustical channel of the earpiece, (4) the acoustic baffle effect of the wearer's person, (5) clothing covering the hearing aid, and (6) the nature of the sound field. These effects have been investigated by techniques described in an earlier paper. In the order listed, it is found that, (1) in many cases the 2-cc coupler does not represent the ear as well as might be desired, (2) a loosely fitting earpiece will reduce the response at low frequencies and will allow some earphones to distort low frequency signals, (3) differences among about 40 commercial earpieces from several manufacturers did not cause important differences in the response, (4) the body-baffle effect consists of several characteristic peaks and valleys in the response, the effect being less marked in a random field than in free field, (5) the effect of clothing over the instrument is to reduce the response in the frequency range 800–3000 c.p.s. and (6) differences between field and pressure sensitivities of the ear are of sufficient magnitude to be considered when comparing performance of aided and unaided ears.