Spiegel

Dispersion

“The degree to which numerical data lend to the spread about an average value is called the variation or dispersion of the data”

Standerd Deviation  σ

“The Standard Deviation is the square root of the arthmatic mean of the square of all deviations. Deviations being measured from the arthmatic mean of the items”

 Index Number

“An index number is a statistical measure designed to show changes in a variable or a group of related variables with respect to time, geographical locations, or other characteristics.”

About the Above Author