The NIST Reference on Constants, Units and Uncertainty

Fundamental Physical Constants
Constants
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Correlation coefficient between two constants
neutron mass in u
 $m_{\rm n}$
 Numerical value  1.008 664 915 95 u
 Standard uncertainty  0.000 000 000 49 u
  Relative standard uncertainty  4.8 x 10-10
 Concise form  1.008 664 915 95(49) u    
 
Source: 2018 CODATA     
recommended values 		 Definition of
uncertainty 		       Correlation coefficient with
any other constant

 
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