Adjusting hearing aid user's real ear performance by using probe-microphone technology (real ear measurement, REM) has been a well-known procedure over 30 years among audiologists. With this measurement technique, it is possible to verify whether the output of the hearing aid at the eardrum matches the desired prescribed target. Still less than half of audiologists verify their hearing aid fitting to match the prescribed target amplification with this technology. Many still rely on the manufacturer's default "first-fit" settings (initial fit approach) which means that the patient's hearing thresholds at any given frequency are transferred to the programming software that predicts the output and gain of the hearing aid by using proprietary or modified prescriptive algorithm. These proprietary algorithms create an approximation over patients in situ hearing aid gain and output based on data such as the age of the patient, an estimate of microphone location effects, the ear mold or shell design and length, venting size, and tubing characteristics.

Recent studies have demonstrated failures to match the prescribed amplification targets, using exclusively the predictions of the proprietary software. The American Speech-Language-Hearing Association (ASHA) and American Academy of Audiology (AAA) have created Best Practice Guidelines that recommend using real-ear measurement (REM) over initial fit approach in order to verify the prescribed gain and output of the hearing aids. Accordingly, the recent ISO 21388:2020 on hearing aid fitting management recommends the routine use of REM. So why is REM still rarely applied clinically? The main reason is the lack of proof over cost-effectiveness and patient outcome. There are only few publications of varying levels of evidence indicating benefits of REM-fitted hearing aids with respect to patient outcomes that include self-reported listening ability, speech intelligibility in quiet and noise and patients' preference. According to a very recent systematic review and meta-analyses by Almufarrij et al. published in 2021, there are only six publications that meets the inclusion criteria, and the evidence favors REM fitting for all outcomes reported (self-reported listening ability, speech intelligibility in quiet and noise and preference). Still, the quality of evidence varies across the outcomes since all articles had a rather limited number of participants and only two used power calculation to determine the sample size. None of these studies reported health-related quality of life, which was assessed to be the primary outcome by the reviewers. Also, secondary outcomes of interest including adverse events, generic quality of life and cost-effectiveness were not assessed. The authors also acknowledged the lack of sufficient follow-up duration (the maximum duration was only 6 weeks) and the lack of permission for further adjustment to the amplification characteristics. In addition, the included studies failed to investigate first-time users over experienced hearing-aid users and the amplification characteristics the experienced users were familiar with, were not reported. This was judged to possibly impact on short-term outcomes since changes of hearing-aid users' amplification characteristics that they are already accustomed to, can cause discomfort. The authors also claimed that future studies should also estimate the importance of any benefit found and evaluate the reasons why participants are reporting these benefits.

In summary, current evidence indicates that the initial fit approach often fails to achieve the prescriptive acoustic gain and output of hearing aids, however, evidence which would clearly show that REM-based hearing aid fitting (which is time-consuming) is clinically relevant and cost-effective is lacking, and thus warrants further studies.

Our main research question is whether REM-based fitting improves the patient reported outcome measures - PROMs (SSQ, HERE) and performance-based outcome measures (speech-reception threshold in noise) over initial fit approach. These are the primary outcomes of our study. An additional research question is whether REM-based fitting improves hearing aid usage (self-reported \& log-data report). Eventually, the investigators will calculate the cost-effectiveness of REM-based fitting. These are the secondary outcomes of our study.