A sigma value is used to express standard deviation, or how far off from the mean the data point is. Sigma symbols are commonly used to express defects per million occurrences. If you want to make a sigma sign on your keyboard, you can do so in a few simple steps.
Position your cursor on the area in the document where you want to insert the Sigma symbol.
Click 'Insert' on the word menu.
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Click "Symbol" on the Insert Menu. A pop-up box will appear to show you all the different symbols that are available. If you do not see the symbol you want in the display, choose "more symbols."
Click on the Greek symbol "σ" to insert it into the document. The pop-up box will disappear and the Sigma symbol will be inserted at the position of your cursor.