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

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:

  • $\begingroup$ This is the PDF with the formula: core.ecu.edu/geology/rigsbyc/rigsby/Sedimentology/… (feel free to edit in to main question :)) $\endgroup$ – Metasomatism Apr 1 '16 at 4:00
  • $\begingroup$ 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. $\endgroup$ – daniel.heydebreck Apr 1 '16 at 11:57
  • $\begingroup$ @daniel.neumann Ahh. I'm a bit silly... So my actual number should be 0.004545455? $\endgroup$ – Metasomatism Apr 1 '16 at 15:45
  • $\begingroup$ 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. $\endgroup$ – daniel.heydebreck Apr 1 '16 at 16:57
  • $\begingroup$ @daniel.neumann That -1.2 should be -0.8? But yes, I'd inverted them all, not just the 5 95. Thanks! $\endgroup$ – Metasomatism Apr 1 '16 at 18:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.