# What is a histogram?

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Connexions module: m13423 1 What is a histogram? ∗ Brandon Hodgson This work is produced by The Connexions Project and licensed under the Creative Commons Attribution License 2.0 † 1 What is a histogram? A histogram is a summary graph showing distribution of data points measured that falls within various class-intervals. A class interval is a division of a range of values into sets of non-overlapping intervals for plotting a histogram. It is drawn with rectangles side by side with the area of each rectangle being proportional to the frequency of the observations falling into the corresponding class-interval. Example 1 Measurements of subscriber behaviour can be represented in a histogram. When subscriber A calls subscriber B the call may take on the following behaviour: • The call may have no answer from subscriber B, • Subscriber B might be busy on the phone, thus signalling busy tone to subscriber A • Subscriber B may be able to answer the call. Thus for a particular exchange one can plot a histogram of failed call attempts which were repeated within 7 minutes when subscriber B is busy. Example 2 This example is extracted from (Wikipedia 2006). All table data and figures are also taken from (Wikipedia 2006). Consider data collected by the U.S. Census Bureau on time to travel to work (2000 census, Table 5). Actually, this document shows bar graphs, but they are not histograms since the bars are not adjacent. The census found that there were 124 million people who work outside of their homes. People were asked how long it takes them to get to work, and their responses were divided into categories: less than 5 minutes, more than 5 minutes and less than 10, more than 10 minutes and less than 15, and so on. The tables shows the numbers of people per category in thousands, so that 4,180 means 4,180,000. The data in the following tables are displayed graphically by the diagrams below. An interesting feature of both diagrams is the spike in th

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