<p>I'm calculating the inclusive graphic standard deviation for sediment from a cumulative frequency graph, with this formula:</p>

<p><a href="https://i.sstatic.net/kIyobm.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/kIyobm.png" alt=""></a></p>

<p>But when I plug in the numbers, I get a negative value, because my ranges at 5, 16, 84 and 95 span the 0φ boundary. What does this mean? Does this mean it is very well sorted, or have I made a calculation error?</p>

<p>My data:</p>

<p><a href="https://i.sstatic.net/4OdKc.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/4OdKc.png" alt=""></a></p>

CC BY-SA 3.0
2016-04-01T04:00:49.403Z
This is the PDF with the formula: http://core.ecu.edu/geology/rigsbyc/rigsby/Sedimentology/CalculationOfGrainSizeStatistics.pdf (feel free to [edit] in to main question :))
Metasomatism
CC BY-SA 3.0
2016-04-01T11:57:36.637Z
Maybe I do not understand the question: According to pages 2 and 3 of the document, negative Phi values are not unexpected. The order of the Q values needs to be inverted: The 95% percentile needs to correspond to the lowest Phi value.
daniel.heydebreck
CC BY-SA 3.0
2016-04-01T15:45:25.867Z
@daniel.neumann Ahh. I'm a bit silly... So my actual number should be 0.004545455?
Metasomatism
CC BY-SA 3.0
2016-04-01T16:57:28.287Z
How do you get 0.004545455? According to the plot on the right, one of the columns in the table on the right needs to be inverted - meaning that Q95 = 2.2, Q84 = 1.8, Q16 = 0 and Q5 = -1.2. Putting these values in the formula we get `(1.8+0)/4 + (2.2-(-1.2))/6.6 = 0.45 + 1.7/3.3 = 0.9652`. Thus, it is moderately sorted.
daniel.heydebreck
CC BY-SA 3.0
2016-04-01T18:01:08.970Z
@daniel.neumann That -1.2 should be -0.8? But yes, I'd inverted them all, not just the 5 95. Thanks!
Metasomatism
<geology><rocks><sedimentology>
What does a negative value for the inclusive graphic standard deviation mean?
Metasomatism
# What does a negative value for the inclusive graphic standard deviation mean?

(Asked by: username_0 on 2016-04-01)

I'm calculating the inclusive graphic standard deviation for sediment from a cumulative frequency graph, with this formula:

[![](https://i.sstatic.net/kIyobm.png)](https://i.sstatic.net/kIyobm.png)

But when I plug in the numbers, I get a negative value, because my ranges at 5, 16, 84 and 95 span the 0φ boundary. What does this mean? Does this mean it is very well sorted, or have I made a calculation error?

My data:

[![](https://i.sstatic.net/4OdKc.png)](https://i.sstatic.net/4OdKc.png)

**Tags:** geology, rocks, sedimentology

> **Comments:**
> **username_0**: This is the PDF with the formula: http://core.ecu.edu/geology/rigsbyc/rigsby/Sedimentology/CalculationOfGrainSizeStatistics.pdf (feel free to [edit] in to main question :))  
> **username_1**: Maybe I do not understand the question: According to pages 2 and 3 of the document, negative Phi values are not unexpected. The order of the Q values needs to be inverted: The 95% percentile needs to correspond to the lowest Phi value.  
> **username_0**: @daniel.neumann Ahh. I'm a bit silly... So my actual number should be 0.004545455?  
> **username_1**: How do you get 0.004545455? According to the plot on the right, one of the columns in the table on the right needs to be inverted - meaning that Q95 = 2.2, Q84 = 1.8, Q16 = 0 and Q5 = -1.2. Putting these values in the formula we get `(1.8+0)/4 + (2.2-(-1.2))/6.6 = 0.45 + 1.7/3.3 = 0.9652`. Thus, it is moderately sorted.  
> **username_0**: @daniel.neumann That -1.2 should be -0.8? But yes, I'd inverted them all, not just the 5 95. Thanks!  

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## 0 answers

