Many biostatistical analyses are conducted to study the relationship between two continuous or ordinal scale variables within a group of patients.

Purposes of these analyses include:

1. assessing correlation between the two variables, i.e., identifying whether values of one variable tend to be higher (or possibly lower) for higher values of the other variable;
2. assessing the amount of agreement between the values of the two variables, i.e., comparing alternative ways of measuring or assessing the same response;
3. assessing the ability of one variable to predict values of the other variable, i.e., formulating predictive models via regression analyses.

This lesson will focus only on correlation and agreement, (issues numbered 1 and 2 listed above).