Why are the last two digits of the mass of an electron in parentheses?

[Mason Smith]

According to Wikipedia, the mass of an electron is 0.510 998 9461(31) MeV/c². Why are the last two digits of this number in parenthesis? Is this the same thing as saying (0.510 998 9461 ± 0.000 000 0031) MeV/c².

 

[Gene Naden]

± 0.000 000 0031

No, it does not mean that. It means that it is approximately equal to 0.510 998 946131 but the last two decimal places are uncertain.

 

[Lord Jestocost]

Use of concise notation (from https://physics.nist.gov/cgi-bin/cuu/Info/Constants/definitions.html)

If, for example, y = 1 234.567 89 U and u(y) = 0.000 11 U, where U is the unit of y, then Y = (1 234.567 89 ± 0.000 11) U. A more concise form of this expression, and one that is in common use, is Y = 1 234.567 89(11) U, where it understood that the number in parentheses is the numerical value of the standard uncertainty referred to the corresponding last digits of the quoted result.

 

[Gene Naden]

OK if what @Lord Jestocost said is true then I was wrong.

 

[Mason Smith]

Use of concise notation (from https://physics.nist.gov/cgi-bin/cuu/Info/Constants/definitions.html)

If, for example, y = 1 234.567 89 U and u(y) = 0.000 11 U, where U is the unit of y, then Y = (1 234.567 89 ± 0.000 11) U. A more concise form of this expression, and one that is in common use, is Y = 1 234.567 89(11) U, where it understood that the number in parentheses is the numerical value of the standard uncertainty referred to the corresponding last digits of the quoted result.

This helped a lot. Thank you for the reference.