NRR - Number rating system representing the average noise reduction in decibals that a hearing protection device will provide. The EPA requires that all hearing protection products have an NRR label.
Unfortunately, the testing process for NRR is conducting in a labratory in a controlled setting where test subjects are fitted by lab technicians. In the real world, workers fit hearing protection devices themselves in less stable and ideal conditions.
To compute a noise reduction that is more realistic, OSHA recommends that you derate hearing protection by reducing the published NRR by 50%.