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

CC BY-SA 4.0
2019-04-10T18:04:29.887Z
2019-04-10T18:04:29.887Z
ProfRob
<p>As far as I know, the plus-or-minus sign is used to indicate the <a href="https://en.wikipedia.org/wiki/Plus-minus_sign#In_statistics" rel="nofollow noreferrer">statistical margin of error</a> of a measurement. But what does it mean when there are two plus-or-minus signs in the same measurement?</p>

<p>Some examples from Wikipedia:</p>

<ul>
<li>The mass of <a href="https://en.wikipedia.org/wiki/Messier_87#Supermassive_black_hole" rel="nofollow noreferrer">M87*</a> is given as <code>(6.5 ± 0.2<sub>stat</sub> ± 0.7<sub>sys</sub>) × 10<sup>9</sup> M☉</code> (as measured by the Event Horizon Telescope).</li>
<li>The measured distance to <a href="https://en.wikipedia.org/wiki/Sagittarius_A*" rel="nofollow noreferrer">Sagittarius A*</a> is given as <code>7860 ± 140 ± 40 pc</code>.</li>
</ul>

CC BY-SA 4.0
2019-04-10T17:57:12.680Z
Look at the subscripts:  it's attribution of portions of the total error to different effects
Carl Witthoft
CC BY-SA 4.0
2019-04-10T18:57:10.293Z
Hm. So are the different portions additive? e.g. could you say that the distance to Sgr A* is 7860 ± 180 pc?
ahiijny
<distances><mass><units><event-horizon-telescope>
What does it mean when there are two plus-or-minus signs for a measurement?
ahiijny
# What does it mean when there are two plus-or-minus signs for a measurement?

(Asked by: username_0 on 2019-04-10)

As far as I know, the plus-or-minus sign is used to indicate the [statistical margin of error](https://en.wikipedia.org/wiki/Plus-minus_sign#In_statistics) 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\*](https://en.wikipedia.org/wiki/Messier_87#Supermassive_black_hole) is given as `(6.5 ± 0.2stat ± 0.7sys) × 109 M☉` (as measured by the Event Horizon Telescope).
* The measured distance to [Sagittarius A\*](https://en.wikipedia.org/wiki/Sagittarius_A*) is given as `7860 ± 140 ± 40 pc`.

**Tags:** distances, mass, units, event-horizon-telescope

> **Comments:**
> **username_1**: Look at the subscripts:  it's attribution of portions of the total error to different effects  
> **username_0**: Hm. So are the different portions additive? e.g. could you say that the distance to Sgr A* is 7860 ± 180 pc?  

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## 1 answers

### Answer by username_2 on 2019-04-10. Score: 3

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


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