Proportions or prevalence estimates are very useful in epidemiological studies. For a national cross-sectional survey such as NHANES, you often need to generate prevalence estimate of a particular disease, condition, or risk factor in U.S. population. It is also used to compare prevalence rates between different subgroups.
In this example, to determine the prevalence rate of high blood pressure in the U.S., you will identify persons who have high blood pressure according to the conventional health care definition set out by the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure. According to the Committee, a person with hypertension is defined as either having elevated blood pressure (systolic pressure of at least 140 mmHg or diastolic of at least 90 mmHg) or taking antihypertensive medication.