Standard Deviation, often abbreviated SD, is a measure of glucose variability.

From "Pumping Insulin", by John Walsh and Ruth Roberts, page 19:

"Emphasis is rightly placed on high blood sugar levels as a cause for damage, but evidence also points to up and down or unstable readings as an additional culprit."

Standard deviation is obtained from insulin pumps, and some glucose meters (including cgm) and the programs that the meters and pumps upload their data to.

What are we measuring here? We are measuring how tightly packed your readings are around your average, or to state the opposite, how widely scattered they are.

If you yo-yo all day from high to low and back, that will show up as a high standard deviation. If you are very stable, without many highs or lows, that will show up as a very low SD.

The lower the number the better.

What is a good number? Walsh says get it under 65 mg/dl (3.6 mmol), or under half your average glucose for a type 1.

As a type 2, I have found that it is easy for me to do much better than that. I started pumping at a SD of about 25 mg/dl (1.4 mmol). Now, 7 months later with no highs and very few lows, my SD is 15 mg/dl (.83 mmol).

Say your average is made up from a 250 (13.9) and a 50 (2.8) averaging 150 (8.3). The 150 (8.3) average is not bad, but the 50 (2.8) is too low and the 250 (13.9) is too high. So if you only look at the average, you might think you are doing well, when in fact you are not doing so well.

Looking at a standard deviation will tell you if your average is made up of good numbers (great!) or high numbers and low numbers averaging themselves out (not good).