A histogram is a graph that shows how many data points fall into specified categories. For example, this histogram shows the number of Systolic Blood Pressure readings (mmHg) in seven categories (or bins) that cover the range of the data.
The data points in this case are one person's blood pressure readings taken over a period of months. The histogram shows that most of the time, the blood pressure falls in the 126-132.9, 139-144.9, and 145-150.9 categories.
Histograms are often used by researchers to help them quickly "see" how points in a large data set are distributed over the range of data.
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