Now that you have determined what variables the study contains, you find yourself left with a collection of numbers, which are the difference in patient’s blood pressure readings after they began taking their pill (medication or placebo) compared with their baseline blood pressure. This collection of numbers is called your "data set".

For the qualitative variables, nominal and ordinal, the numbers are an indicator of the number of patients that belong to each category (i.e. male or female, stage I or stage II).

For the quantitative variables, the numbers are the actual numerical measure of the variable in question (i.e. blood pressure difference).

Quantitative variables can be further classified as:

• Continuous data: can be measured to as many decimal places as the measuring instrument allows
• Discrete data: there are only a finite number of values possible (compared to continuous where there is an infinite number of possible values)