Quite right XBuiler, but the hearing aid fitting (manufacturer’s) software does know what the aid is capable of and uses averages to determine the fitting. The problem with the average setting (first fit) is that is suits only a very small percentage of people’s hearing loss well. Most people’s ears are not average. When REM is done we measure what the natural ear canal resonance is (the ear canal boosts certain frequencies more than others due to canal shape, size consistency etc.), when you place something in this ear canal space you have to accurately compensate for what you take away (by using some of this space to fit the hearing aid which can be at different depths, have different venting etc.) and then correct for the hearing loss at the same time. Any hearing aid, even if set too softly will create an impression of working and may sound quite nice, but WILL NOT neccesarily give you the correction needed to correct for intelligibility without the help of REM. This can lead to a number of issues as a hearing aid does not fix your hearing, it compensates for the specific damage to your ear, stimulating the brain with the corrected information (We hear with the Brain not just the ear). Your brain then, over time, learns to make sense of this corrected information and forms new neural connections based on this correction as long as you use the aids consistently (which is why people wearing their aids more thend to do much better than people who don’t). If this has not been done properly, you may run a risk of forming new neural connections based on incorrect sounds and not find much benefit in the long run (or may not be able to accept a properly fitted aid in the future as easily), once the honeymoon period (1st 3 months or so) has passed. If the hearing aid is then set correctly at a later date the brain has to retrain to gain the missed info (this ability tof the brain to form new connections (neural plasticity) gets worse as we get older -which is why someone who addresses their hearing loss earlier tends to do better than someone who addresses it much later). There is also a slight risk of sensory deprivation if the aid does not compensate for the sound it takes away from the natural ear canal resonance, meaning the hearing aid actually makes you hear certain areas sounds WORSE with the hearing aid than without. This is turn may switch of some of the neural connections you had before using the hearing aid. So fitting your own aids or not having REM or other verifications done at fitting can be risky not only for this reason, but a few others not mentioned here.