How to Compute Vector Means
Mean Scores: Vectors
In ordinary algebra, the mean of a set of observations is computed by adding all of the observations and dividing by the number of observations.
x = Σxi / n
where x is the mean of observations, Σxi is the sum of all observations, and n is the number of observations.
In matrix algebra, the mean of a set of n scores can be computed as follows:
x = 1'x ( 1'1 )-1 = 1'x ( 1/n )
To show how this works, let's find the mean of elements of vector x, where x' = [ 1 2 3 ].
|x =||[ 1 1 1 ]||
|(||[ 1 1 1 ]||
x = 6/3 = 2
Thus, the mean of the elements of x is 2.
Mean Scores: Matrices
You can think of an r x c matrix as a set of c column vectors, each having r elements. Often, with matrices, we want to compute mean scores separately within columns, consistent with the equation below.
Xc = Σ Xic / r
is the mean of a set of r scores from column c
Σ Xic is the sum of elements from column c
In matrix algebra, a vector of mean scores from each column of matrix X can be computed as follows:
m' = 1'X ( 1'1 )-1 = 1'X ( 1/r )
m' is a row vector of column means,
1 is an r x 1 column vector of ones
X is an r x c matrix of scores: X11, X12, . . . , Xrc
The problem below shows how everything works.
Test Your Understanding
Consider matrix X.
Using matrix methods, create a 1 x 3 vector m', such that the elements of m' are the mean of column elements from X. That is,
m' = [ X1 X2 X3 ]
where Xi is the mean of elements from column i of matrix X.
To solve this problem, we use the following equation: m' = 1'X ( 1'1 )-1. Each step in the computation is shown below.
|m' =||[ 1 1 ]||
|(||[ 1 1 ]||
|(||[ 1 1 ]||
Thus, vector m has the mean column scores from matrix X. The mean score for column 1 is 6, the mean score for column 2 is 3, and the mean score for column 3 is 2.5.