I am looking for some advice for abbreviating "standard deviation" when it is used in an informal sense as a mathematical unit, especially with regard to financial usage. This often crops up when the unit of measure is measured in another quantity.
I note a previous question 1 seemed to be asking for capitalisation advice, presumably within normal text.
An example would be, "the typical bid-offer spread of Heating Degree Day futures is 0.7 [Standard Deviations]"
Standard sources (Greenbook.org or the APA style guide) favour or "sd" or "Std. Dev.". The first looks to have the right typographical features but just doesn't feel sufficiently well used to be a standard (at least in this application). The second has the drawback of punctuation within a unit, which looks undesirable. It could be further reduced to "stddev" but the double d looks not quite right. I have seen others use the Excel function name, with capitalisation, "STDEV" but this looks too accommodating to Excel users. Perhaps the most satisfactory would be the simple lowercase Greek letter, sigma, which I think would be likely understood by most relevant readers. However, this limits the text being readily electronically copied.
I would be grateful for any opinions on the subject.