   - parameter values: Array of decimal numbers.
   - returns: Population variance. Returns nil when the array is empty.
   
-  Formula
+  Formula:
   
       σ^2 = Σ( (x - m)^2 ) / n
   

  - parameter values: Array of decimal numbers.
  - returns: The median value from the array. Returns nil for an empty array. Returns the greater of the two middle values if there is an even number of items in the array.

  Example

      Sigma.medianHigh([1, 12, 19.5, 10, 3, -5]) // 10
   

  - parameter values: Array of decimal numbers.
  - returns: The median value from the array. Returns nil for an empty array. Returns the smaller of the two middle values if there is an even number of items in the array.

  Example:

      Sigma.medianLow([1, 12, 19.5, 10, 3, -5]) // 3
   
