As the definition of most health related events depends on more than one piece of information or characteristic, it is often necessary to combine more than one variable into a composite variable. If the collected variables are quantitative the created variable can be either quantitative (as body mass index can be computed from weight and height) or qualitative (as hypertensive and non-hypertensive can be computed from systolic and diastolic blood pressure). If qualitative variables are used only a qualitative variable can be computed (as a definition of disability includes children who have visual, auditory or mental disability.