The number of Degrees of
Freedom is the number of independent data
values that are used in estimating the value
of a population parameter.
The number of degrees of freedom in the
deviation formula is ‘n-1’:
If 'n' were used, instead
of 'n-1', the value of 's' would be biased;
the standard deviation calculated from small
samples would underestimate the population
The number of degrees of freedom
is 'n-1' because only 'n-1' of the data
values 'xi' are independent;
if any 'n-1' are known then the other can
be calculated (using x-bar).