Statistics and Data Analysis for Microarrays Using R and Bioconductor, 2nd Edition

Elements of statist ics 217

=

P

n

i=1

X

i

n

−

n

X

n

=

X −X = 0 (8.12)

This difficulty can be overcome in two ways: by taking the average of the

absolute values of the devia tions or by using their squared values. It turns out

that the most useful measure has the following e xpression:

σ

2

=

P

N

i=1

(X

i

− µ)

2

N

(8.13)

This is called popul ation variance and is a measure that characterizes

very well the amount of variability of a population. The variance is usually

reported w ith two more decimal places than the data and has as measure-

ment units the square of the meas urement units of the data. This is actually

the average square distance from the mean. Note that this is