Standard Deviation
A statistical term that expresses the variability in a data set. It is indicated by the Greek letter sigma, and calculated in 3 steps:
- Estimate the average (mean) of the data.
- Find the difference between each data and the mean, and then square each difference.
- Sum all squared differences.
- Divide the sum by the number of data minus 1.
- Calculate the square root of the above.
68% of the data will fall within one standard deviation of the mean. For a standard deviation of 2, this value goes up to 95%, whereas for 3 the standard deviation is 99.7%.