SD?

Standard Deviation.

How To Calculate Standard Deviation
First, you need to determine the mean. The mean of a list of numbers is the sum of those numbers divided by the quantity of items in the list (read: add all the numbers up and divide by how many there are).
Then, subtract the mean from every number to get the list of deviations. Create a list of these numbers. It's OK to get negative numbers here. Next, square the resulting list of numbers (read: multiply them with themselves). Add up all of the resulting squares to get their total sum. Divide your result by one less than the number of items in the list. To get the standard deviation, just take the square root of the resulting number

I know this sounds confusing, but just check out this example:
your list of numbers: 1, 3, 4, 6, 9, 19
mean: (1+3+4+6+9+19) / 6 = 42 / 6 = 7
list of deviations: -6, -4, -3, -1, 2, 12
squares of deviations: 36, 16, 9, 1, 4, 144
sum of deviations: 36+16+9+1+4+144 = 210
divided by one less than the number of items in the list: 210 / 5 = 42
square root of this number: square root (42) = about 6.48
Click to expand...


If you use something like Caltech Cube Timer it will automatically do it for you.
 
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A low standard deviation only means the the times are very close to one another. If you're looking for consistency, then a low SD is good.

Edit: Wow, Kian, great minds think alike.
 
It is better to get a low Standard Deviation, it means you are much more consistent with your times. It is not necessary though.
For example, a low standard deviation would be getting an average of 30 seconds where all your times were between 29-31, and high standard deviation I'm sure you can work out for yourself.
I would much rather have a low standard deviation, it would mean if someone asked "How fast can you solve it?", which I get quite often, and I said "20 seconds" then it would mean I am much more likely to get that time and not be accused of lying!
It would also mean that you can correct and target faults easier, if you are doing something consistently the you can change it and see the difference!
 
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