At present, no published formula is sufficiently precise to be considered reliable under any circumstances. However, some formulas have been found to be more reliable than others.
Gellish's formula is the result of 25 years of research at the University of Okland (Michigan) Medical Institute. This formula has been found to be the most reliable because it has a margin of error of ± 6 to ± 10 beats per minute, which falls to ± 0 to ± 2 bpm between 35 and 65 years of age. It is therefore a reliable reference for calculating Maximum Heart Rate between 30 and 75 years of age.