Range It is the difference between highest and lowest value in the data set. Range provides some idea about the variability in the data (more than the mean alone). The range could be misleading if one of the extreme value is atypical.
Interquartile Range (IQR) It is the better measure of dispersion than the Range. It uses the terms Q1, Q2, Q3, Q4 to denote each quarter of the data set. Q1 lies at the 25th percentile, Q2 lies at the 50th percentile or median, Q3 lies at the 75th percentile, and Q4 lies at the 100th percentile. The IQR is obtained by Q3 - Q1.i.e., the value at the 75th percentile minus the value at the 25th percentile.
Standard Deviation (s) It is the better measure of dispersion compared to range and IQR because unlike range and IQR, the Standard deviation utilizes all the values in the data set in its calculation. The square of the standard deviation is called Variance(s2). Following is formula for computing standard deviation for population or sample data. The differnce between two values becomes small as 'n' becomes large. It is necessary to substract 1 from 'n' because it provides the correction for the bias, i.e., the sample has less variation or standard deviation than the population.