Standard Deviation

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  The standard deviation is a statistic that tells you how tight or loose the range/spread of data is in relation to the mean or average of the data.  Typically when a data range is clustered tightly the "bell curve" will be tall/steep.  When the data is range is spread far apart the "bell" will appear flat or short and wide.  

Example of Spread:

As you can see here; A low standard deviation shows the cluster very close to the mean. The higher the standard deviation the more the values spread apart from the mean.

How to Calculate Standard Deviation:

1. Compute the process average.
2. Subtract the process average from each data value.
3. Square each deviation that you calculated in step 2.
4. Add all the squared deviations from the third step.
5. Divide the result of step 4 by the sample size.
6. Take the square root of step 5’s result.

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Formulas in Google Sheets:
  Remember that all formulas begin with an equal sign inside of your cell in google sheets.

• =STDEV: calculates standard deviation for a sample. To calculate standard deviation across an entire population, use STDEVP.
• =STDEVPA: Calculates the standard deviation based on an entire population, setting text to the value `0`.
• =STDEVP: Calculates the standard deviation based on an entire population.
• =STDEVA: Calculates the standard deviation based on a sample, setting text to the value `0`.

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Ref: Google Docs Editor Help - STDEV

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