Dispersion is the measure of variation of items. It measures the extent to which the items vary from central value. Dispersion is also known as average of the second order.
In the words of Brooks and Dick,” Dispersion is the degree of the scatter or variation of the variable about a central value.”
In the words Simpson and Kafka,” The measurement of the Scatterness of the mass of figures in a series about an average is called measure of variation or dispersion.”
Dispersion includes range, mean deviation, quartile deviation, and standard deviation.
Mean, Median and Mode are the average of 1st order.
Purpose of Measure of Dispersion: Measures of Dispersion are needed for the following purposes:
1. To determine the reliability of an average.
2. To serve as a basis for the control of the variability.
3. To compare two or more series with regard to their variability.
4. To facilitate the use of other statistical measures.
Classification of Measure of Dispersion
Measure of dispersion may be broadly classified into two types:
1. Absolute measures of dispersion: It is classified into
i. Range
ii. Mean Deviation
iii. Standard Deviation
iv. Quartile Deviation
2. Relative measures of dispersion: It is classified into
i. Coefficient of Range
ii. Coefficient of Mean Deviation
iii. Coefficient of Variation
iv. Coefficient of Quartile Deviation.
Difference between Absolute and Relative Measure of Dispersion:
1. Absolute measures are dependent on the unit of the variable under consideration whereas the relative measures of dispersion are unit free.
2. For comparing two or more distributions, relative measures and not absolute measures of dispersion are considered.
3. As compared to absolute measures of dispersion, relative measures of dispersion are difficult to compute and comprehend.