Relationship between variables
In most data sets, we are interested in the relationships between the variables.
For bivariate numerical data, correlation and least squares give information about the relationship. A contingency table and conditional proportions summarise the relationship between two categorical variables. If a numerical variable is recorded from individuals in two or more groups, differences between the groups can also be expressed as relationships between the variable and a categorical variable describing the grouping.
Interpreting relationships
Relationships can be much harder to interpret than you might think. In some situations, the relationship between two variables, such as the relationship evident in a scatterplot, may not describe a meaningful 'real' relationship.