A parameter that indicates the way in which a probability function or a probability density function is centred around its mean.

In other words how far away (on average) a result is from the mean.

To calculate:
  • Add together all items
  • Divide the total by the number of items
  • This is the mean
  • Subtract each item from the mean
  • Multiply each result by itself
  • Add all these results together
  • Divide by the total number of items
  • This is the Variance
  • The square root of the Variance is the standard deviation
The smaller it is (in proportion to the mean) the more likely the mean is to be a useful result and the less likely there are to be a few very high or very low results making the mean a bad representation of the data.