Some characteristics are subject to inherent variation from use conditions: for example, I can imagine that you need to prove that the hearing aid meets output requirements given a range of sound volume and frequency as well as ‘fit’ in the ear? In this case your sample size is more dependent on the range of each condition than anything else. The number of devices is less important than the number of conditions. Do you just test at the extremes and the nominal? Or across the range in a distribution of conditions that is representative of the conditions that exist in actual use? It is also dependent on whether or not the hearing aid is consistent in its performance across the range. In this case of design verify you can usually have a small number of devices tested across a full range of conditions. I typically would recommend 3 devices just to be safe. The ‘number of conditions tested on each device’ is then the real sample size.