The Hearing Aid Fitting procedure contains a series of measurements during the Assessment and Verification steps. In order to validate the outcomes of the Hearing Instrument Fitting and to increase the level of confidence of the responsible Audiologist or Hearing Aid Professional the aforementioned phases have to be performed under a ‘high accuracy’ perspective. Corrections, conversions and calculations on different aspects of the sound signal as well as psychophysical, psychometric and Human Dynamics are in course of the study of the Hearing Instrument Fitting where complex audiometric diagnostic testing and electroacoustic measurements are implemented. The status of the instrumentation, the application of the accessories (transducers, microphones, etc.) and the condition of the examined individual can influence dramatically the results of hearing amplification. Several studies have examined the potential errors of these phases as separate entities but very few have targeted the overall procedure. The goal of this study is to bring in the foreground the possible errors in all stages of the Hearing Instrument Fitting procedure and raise awareness of those important factors that can influence the outcomes of fitting in terms of hearing amplification benefits and user satisfaction.