Standard Deviation
For sample
 x  x 
2
S.D =
n 1
Variance for grouped data
For sample
S.D = s
2
 f x  x

i
2
i
n 1
MEAN DEVIATION
The mean deviation of a set of data is defined as the arithmetic mean of the deviations
measured either from the mean or from the median.
For sample
M .D 
1 n
| x  x |
n i 1
For a population
M .D 
1
N
N
| x   | ,
i 1
QUARTILE DEVIATION
Quartile Deviation is the average of 1st and 3rd Quartile.
Q.D = (Q3 – Q1)
2