Standard Deviation: What Is It and Why Is It Important?

Oh Shit MeterYesterday when I wrote about my A1c, I mentioned that my standard deviation for the period covering that A1c was 62.3. I also stated that was a lot of bouncing around.

Usually when the subject of standard deviation comes up, at least one person will ask what standard deviation is so I thought I would first explain what it is. In simple terms, a standard deviation measures how much bouncing around your blood sugar does. The more glucose excursions you go on, the higher your standard deviation will be. If your blood sugar is staying very flat, then it will be closer to zero. I have actually had my standard deviation show up as zero on my Dexcom reports overnight. Realistically though, that doesn’t happen during the day for me because of all the things that throw off blood sugar.

Most meters will calculate the standard deviation for you. If you are only testing 4 times a day and usually before you eat, your standard deviation number isn’t really going to mean much because it is not picking up your after meal spikes. The Dexcom records blood sugar every 5 minutes and there are 12 blood sugar readings per hour used to calculate standard deviation. The more blood sugar readings there are, the standard deviation will be more accurate. A CGMS is reading all the hours you spend high along with all the time you spend low.

If you want to know how to do it by hand, you would need to add up all your blood sugar readings to get an average. Then you would subtract each blood sugar from the average and square it. For instance, if your average was 100 and one of the readings was 68, you would subtract 68 from 100 to get 32. You would square 32 to get 1024. You then add up all those square numbers and take the average of the squares. Take the square root of the average of the squares to get your standard deviation. Or you could just plop all your numbers in Excel and use the stdeva formula. But the easiest way is just to have a meter or CGMS that does all the work for you.

I first read about the impact of those glucose excursions on your cardiovascular health in an article on Science Daily. I haven’t been able to find that article again, but a discussion came up on TuDiabetes about the impact of those excursions. Tom posted some good studies both pro and against in that discussion. You can read that discussion here. He also posted a link that contained some other pro and against studies that you can read here.

I personally don’t need a study to tell me that I feel like crap when my blood sugar is bouncing all over the place. Feeling like crap is all the scientific evidence that I need to know something is not right.  Guess which one felt better!

Dexcom sick day

Dexcom 24 hour flatline

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4 thoughts on “Standard Deviation: What Is It and Why Is It Important?

  1. You know, Kelly, I think that the longer we have had db, combined with the older we get, the more sensitive we are to spikes and bounces – It seem like when I was in my 20’s and 30’s, I could run around day and night with my bg at every different level and my energy was good. Sure ain’t the same now….

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