# Multivariate correlations

### Problem statement

In science and technology systems (objects) are characterized by a finite set of parameters: xii = 1, ... , N. Consequently, the measurements for these parameters can be arranged into a rectangular matrix:

 (1)

Here M is the number of experiments. Each experiment corresponds to the measurement of a system , sample, individual, etc. All such terms are used interchangeably.

### Examples

 Systems Parameters Human individuals Age, sex, education, income, weight, height, etc. Chemical solutions Spectral intensities at selected wavelength Microchips in a control sample Voltage and current at certain pins Clinical test participants Lab test results

### Mean values

The sample (population) mean vector of parameters is defined as:

 (2)

For each measurement the vector of deviations can be defined as:

 (3)

In the case of clinical research, one of the components of μ is an average patient temperature in a hospital. Obviously, more interesting is a deviation from this average.

The vectors of deviations form the matrix D similar to the initial matrix X:

### Covariance matrix

The sample covariance matrix is defined as averaged products of the deviation vector components:

Here di,m is the i-th parameter of the m-th system.

Eq (5) can be rewritten in the following matrix form:

 (6)

The superscript "T" denotes the matrix transposition.

### Variances

The maximum likelihood covariance matrix CML differs by the factor M /(M-1) from the above definition:

 . (7)

The advantage of this definition is that the i-th diagonal element is the estimation for the variances of the  i-th parameter:

 (8)

### Correlation coefficient

Regardless of the covariance definition, the correlation coefficients are:

 (9

or in a matrix form:

 (10)

Here is a diagonal matrix with the following matrix elements:

 (1)

The correlation coefficient is a measure of the quality of a linear least squares fit for the original data. A higher σ value means a better linear fit.

This approach is implemented in a program called "Correlations". This program is available in the Download section below. You can also use more general "Stat Analysis" program.

Remark: In "Correlations" the meaning of the columns and rows is opposite to that of the tutorial.

1. Program "Correlations".
2. Program "Stat Analysis".

### References

© Nikolai Shokhirev, 2001 - 2017

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