The selection of participants for usability assessment, together with the minimum number of subjects required to obtain a set of reliable data, is a hot topic in Human Computer Interaction (HCI). Albeit, prominent contributions through the application of different p estimation models argued that five users provide a good benchmark when seeking to discover interaction problems a lot of studies have complained this five-user assumption. The sample size topic is today a central issue for the assessment of critical-systems, such as medical devices, because lacks in usability and, moreover, in the safety in use of these kind of products may seriously damage the final users. We argue that rely on one-size-fits-all solutions, such as the five-user assumption (for websites) or the mandated size of 15 users for major group (for medical device) lead manufactures to release unsafe product. Nevertheless, albeit there are no magic numbers for determining "a priori" the cohort size, by using a specific procedure it is possible to monitoring the sample discovery likelihood after the first five users in order to obtain reliable information about the gathered data and determine whether the problems discovered by the sample have a certain level of representativeness (i.e., reliability). We call this approach "Grounded Procedure" (GP).The goal of this study is to present the GP assumptions and steps, by exemplifying its application in the assessment of a home medical device.