Yop, the nerd in me came out this weekend.....
http://www.mathsisfun.com/data/standard-deviation.html
The "simple" version of it is, how much can you expect the value to go from the mean. If you have an average of 6.2 and a standard deviation of .61, you can expect the fuel mileage to be 5.6 to about 6.8 MPG and that would be reasonable and expected.
I went back over the last 2 years since I bought my truck. Up to April of this year, my Standard Deviation has been about 1.57. My average was for 2012 was 6.7 with a SD of 1.77. In 2013, it was 6.25 and SD of 1.57. I was having issues with CAC, overhead, leaking exhaust, cracked exhaust.
For most of 2014, after the most before I replaced my baro sensor, I was seeing a 6.1 mpg and a SD of .91. And my mileages were for the most part from 5.2 to 6.8.
AFTER the baro sensor went in, My mileage has jumped to 7.1 and my SD has dropped to .61. And that's what I needed to see. Because it tells me that my engine is actually DOING what it's supposed to be doing controls-wise.
Standard deviation is one way of looking at your averages over time and seeing if SOMETHING changed. Good, bad or indifferent. In this case, it's telling me the computer is NOW finally doing what it's supposed to with all of the inputs to control the output and do it efficiently.
Without the engine working correctly, I can do all I want to change my trailer, tires, oils etc. It's pointless until the powerplant is working RIGHT.