Measures of Association
Measures of association quantify the strength and the direction of the relationship between two data sets. Here are the two most commonly used measures of association:

Covariance

Correlation
Both measures are used to show how closely two data sets are related to each other. The main difference between them is the units in which they are measured. The correlation measure is defined to assume values between –1 and 1, which makes interpretation very easy.
Covariance
The covariance between two samples is computed as follows:
The covariance between two populations is computed as follows:
Correlation
The correlation between two samples is computed like this:
The correlation between two populations is computed like this: