Their meta-analysis (of 351 prior studies involving 492 groups and 18,712 subjects) and laboratory study (of 514 healthy subjects) concluded that, using this equation, HRmax was very strongly correlated to age (r = −0.90). The regression equation that was obtained in the laboratory-based study (209 − 0.7 x age), was virtually identical to that of the meta-study. The results showed HRmax to be independent of gender and independent of wide variations in habitual physical activity levels. This study found a standard deviation of ~10 beats per minute for individuals of any age, meaning the HRmax formula given has an accuracy of ±20 beats per minute.