For *n *observations *xl' *x2, .......... ,X and an arbitrary constant A, the *r *th moment about A is defined as,

For simple frequency distribution, the *r *the moment about A is defined as

For grouped frequency distribution, *X; *will be taken as class mark.

These moments, 1-l,.', are also known as ' raw moments'.

When A = *x, *then these moments are called 'central moments' and we denote as follows :

The third and fourth central moments *i.e., *~3 and ~4 are used to measure skewness and kurtosis

which have been given in the following sections.

The important relations between the central and raw moments are as follows :

Note. If the above moments are called as sample moments, then we have to use the notation *m', *instead of *ll'r- *See the method of moments in Chapter 10A.** **