Graphical Presentation Of Tabular Data – Histograms (Part 2 of 5)

This is the second in a series of articles aimed at showing the benefits of presenting tabular data in a graphical format.

The first article explained how to carry out a simple survey which could be completed by children, the results being presented in a tabular form. Although the results could be reorganised to produce tables showing the data in different formats, such as by name in alphabetical order or in ascending order of height, the interpretation of the data is not immediate to the person reading it and conclusions may not be easily drawn. This not only causes difficulties for both the surveyor and the reader, but can be a little discouraging for the surveyor in trying to determine what they have achieved from their efforts.

We therefore need to display the data in an alternative format so that more can be achieved from the data gathered. One way of doing this is to use a graphical representation known as a histogram.

Histogram

A histogram is the best graphical method to use for plotting and displaying continuous data such as height or weight. It is not so good for displaying data known as discrete or discontinuous, where the subject has distinct survey criteria such as hat sizes or colours.

Let us assume that the child’s height survey, already mentioned in article one, consisted of a sample of 25 children all of the same age. The smallest child was 1.105m tall and the tallest height was 1.687m.

The visual representation of data for a histogram is a chart where the data values are represented by vertical columns whose heights are equivalent to each of the values of one of the survey’s criteria and all these vertical heights are proportional to each other. The width of the column is immaterial but is usually the same for each value displayed. It is preferable that the survey data is recorded in ascending order of height so that the smallest child’s height is recorded first.

Using our survey example above, the vertical axis, or as it is more commonly known, the ‘y axis’, would represent the height of each child within the survey and would range from 1.0m to 1.7m in ascending order. All the surveyed height values of 1.105m to 1.687m would be contained within the chart. The horizontal axis, or ‘x axis’, would represent the 25 names of the children in the sample.

For each child’s name a point is plotted on the chart corresponding to the height of the child. A vertical narrow column is then drawn above the child’s name up to the height point. This column can be given its own colour to differentiate it from its neighbours on the chart. All the remaining children’s heights are plotted in a similar way. If two or more children have the same height then the columns are drawn adjacent to each other. This is where using different column colours or patterns makes identification easier.

This simple histogram will be a series of vertical columns, the shortest on the left and the highest on the right. It will be clear from this chart which child is the shortest, which is the tallest and where each child is placed within the survey. It will also show in which height ranges the children’s heights are concentrated. As stated previously, the survey results will be of greater benefit for interpretation if all the children were about the same age or in the same class at school. Different colours could be used for boys and girls to show how their height ranges varied, if at all.

This form of data presentation is more pleasing to the eye and understandable than any of the spreadsheet tables. The person presenting the survey will find their output much more satisfying and should have greater pride in their achievement.

The next article in this series considers the use of vertical column, horizontal bar and cylinder charts. Details will be provided as to when they should be used, how they are constructed and the benefits that they can provide, both to the presenter and their audience.