Average vs. Standard Deviation

6 08 2007

At my pump training a month ago, the pump trainers looked at my blood sugar history. They noted the standard deviation (std dev) was 46 and congratulated me on doing such a good job. Huh? I’m not a math major (that would be GiR), but I know that looking at the std dev alone is not the complete picture; just like looking at the average along is not the complete picture either. Take these examples:

  1. 50, 50, 100, 200
  2. 90, 95, 100, 105, 110

They both have the same average (100), but they have drastically different std devs. #1 has a std dev of 61 and #2 has a std dev of 7. Most glucometers output the average, but you can’t just take that by itself. Clearly a lower std dev indicates better control. However, you can’t just look at std dev by itself either.

  1. 90, 95, 100, 105, 100
  2. 205, 210, 215, 220, 225

We already know that #1 will produce an average of 100 and a std dev of 7. #2 also produces a std dev of 7. Does that mean it is good control? Not really. #2 has an average of 215! Not a place you’d want to be.

All I’m saying is to make sure you take both the average and the standard deviation into consideration. Both numbers together indicate your control. You want an average in your target range and a low standard deviation.

Note: This is the standard deviation calculator I used for this post. It’s a quick little tool that can help provide more data about your blood sugar.

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2 responses

6 08 2007
Christine

My Cozmo pump calculates standard deviation and I think that’s a wonderful feature. I’ve also created an excel sheet that does it.

5 09 2008
Kelsie

Carelink (the software that comes with my Minimed pump) calculates the st. dev. for you. I didn’t know what that meant in context of my blood sugars, so I’m glad I stumbled upon your blog. My SD is about 80 which is probably not good. I’m glad I know that now, so that I can try and work on better control!
So thank you for the information.

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