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4.5 Histograms
If you have too many values in a survey, it makes it difficult to present each individual value. In that case, it is a good idea to group together values in order to show them in a more general manner. Value groups are called classes and the type of diagrams which are used to show them are called histograms.   

Every class has a certain interval and this is called the class width


A histogram is very much like a bar chart but instead each bar is placed directly next to each other without any space in between.





Statistics Sweden (Statistiska Centralbyrån) collects statistics about Sweden’s inhabitants and below shows a table about the age distribution in the country on December 31st, 2010. The class width in the table is 10 years.

This example clearly shows that it becomes difficult to account for each individual value when we have so many values. The age interval is between 0 and 110 years, and the diagram would be very difficult to read with a bar for every year. A histogram where the ages are divided up into class widths of 10 years gives a clearer picture of the distribution. Instead of a bar chart with 110 bars, it is better to make a histogram with 11 bars.



For example, the value 10 years in the table above could be thought of as belonging to the first and the second class. Which of the class should this age belong? There is an important rule of thumb for values which lie between two classes:



In other words, age 10 should belong to the 10-20 class.




A histogram, for example, can look like the following:






Another common way to show age distribution is by using a population pyramid. This is another type of histogram where the bars lie horizontal.

In the example to the right the age distribution is show in Cambodia and Germany. The blue bars show men and the pink women.