Standard deviation

• Standard deviation


  It measures how spread out the data is from the middle
• 1 standard deviation


  68% is withing one standard deviation of the mean
• 2 standard deviations


  95% of the data falls within 2 standard deviations of the mean
• 3 standard deviations

  99.7% of the data falls within 3 standard deviations of the mean
• Normal distribution 


  Standard deviation is used on a normal distribution curve. It peaks at the mean and the graph gets very close to the axis but never touches it at 3 standard deviations away from the mean
• Risk chart


  Standard deviations are used to asses risks in production of items. The graph shows a target line usually a target weight and the the upper warning limit is 2 standard deviations above the warning line and the lower waning limit 2 below. The action limits are 3 standard deviations away from the target line
• Shows

  A high standard deviation means that there is a large spread of data
• Variance


  When calculating standard deviation the final step is to square root it but to get variance just don't do this last step.

  Variance is standard deviation squared.
• Used in other calculations

  Used in:

  • Skew
  • Standardised score