NOS is a validated quality assessment instrument for nonrandomized trials that assesses 3 parameters of study quality: selection, comparability, and exposure assessment. The NOS assigns a maximum score of 4 for selection, 2 for comparability, and 3 for exposure, for a maximum total score of 9. Studies with a total NOS score of 5 or greater are considered to be of moderate to high quality, whereas those with an NOS score of less than 5 are considered low-quality studies.