This is the basis of standard deviation, correlation coefficient and variance.
Say we have 22 measurements of student heights. First we find the mean. Deviation is defined as a “distance” from the mean.
Variance is the simplest idea — average product of deviation*deviation. It has dimension of meter*meter
Standard deviation σ is based on variance — √average product of deviation *deviation. Dimension of meter
Now we pair up student’s height deviation and the same student’s weight deviation.
Correlation coefficient is — (average product of deviationh * deviationw) /(σh * σw) Dimension of …nothing.