Question
I've heard that test-retest reliability is a problem when you use stored equalization with open fittings. Can you comment on this?
Answer
Well, first, it indeed is necessary to use stored equalization (rather than concurrent) when assessing open-canal products. This means that a"pretest" equalization is obtained, stored, and then the reference microphone is inactive during the actual testing. This does have the potential to cause unreliable results, if during the testing the patient moves his head to a different position from where it was located for the pretest measure.
The solution, however, is quite simple. Once you have completed the stored equalization measure, you simply say: "Okay, for the next few seconds, don't move your head. Keep it in that exact location." Using the "don't move your head" technique, Todd Ricketts and I recently collected some test-retest data for OC fittings using stored equalization (Ricketts & Mueller, 2009). After the initial testing, we took off the hearing aid, removed the probe tube and the probe-mic gear, and then started over again, replacing the probe tube, etc—as if it were a different test session. We did this for the Frye Fonix, the MedRx Avant and the Audioscan Verifit, using their 'Open Fitting" protocols and test procedures.
Our data showed standard deviations ranging from 0.5 to 2.9 dB for all the key test frequencies (as expected, the greatest variance was at 4000 Hz);there were no significant differences among the three different systems. I consider these values very acceptable for clinical verification measures, and in fact, they are quite similar to what David Hawkins and colleagues reported using concurrent equalization several years back.
Reference
Ricketts TA, Mueller HG (2009). Whose NAL-NL fitting procedure are YOU using? The Hearing Journal, 62(7),10-17.
Dr. H. Gustav Mueller is Professor of Audiology, Vanderbilt University, and has a private consulting practice nestled between the tundra and reality in Bismarck, ND. He is the Senior Audiology consultant for Siemens Hearing Instruments and Contributing Editor for The Hearing Journal. He also holds faculty positions with Central Michigan University, University of Northern Colorado and Rush University. Dr. Mueller is a Founder of the American Academy of Audiology, serves on the Editorial Boards of several audiology journals, is the Hearing Aids Series Editor for Plural Publishing, and is one of the earGuys at earTunes.com.
More information on verification of open canal hearing aids can be found in Dr. Mueller's recorded course, Open Canal Fittings: Unique Acoustic and Verification Considerations.
