The effectiveness is typically measured as a personal attenuation rating (PAR) which is subtracted from the known noise exposure to estimate the total noise exposure a single person has when wearing the tested hearing protection device (HPD).[1][2] The Occupational Safety and Health Administration and the National Hearing Conservation Association Best Practice Bulletin: Hearing Protection Fit-Testing: Hearing Protection- Emerging Trends: Individual Fit Testing describes existing testing methods and how to incorporate them in hearing conservation programs.