As far as I know, the plus-or-minus sign is used to indicate the statistical margin of error of a measurement. But what does it mean when there are two plus-or-minus signs in the same measurement?

Some examples from Wikipedia:

  • The mass of M87* is given as (6.5 ± 0.2stat ± 0.7sys) × 109 M☉ (as measured by the Event Horizon Telescope).
  • The measured distance to Sagittarius A* is given as 7860 ± 140 ± 40 pc.
  • $\begingroup$ Look at the subscripts: it's attribution of portions of the total error to different effects $\endgroup$ Apr 10 '19 at 17:57
  • 1
    $\begingroup$ Hm. So are the different portions additive? e.g. could you say that the distance to Sgr A* is 7860 ± 180 pc? $\endgroup$
    – ahiijny
    Apr 10 '19 at 18:57

The first error is due to the precision of the measurement, the second is an uncertainty associated with methodology used.


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