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Delete - Last Modified By: BJO at 17/02/2015 10:02:02 AM
Dot Plots are another convenient and quick way of displaying and visualising Univariate data, particularly if the data comes from a limited number of values. The data is arranged so that the possible values form a horizontal number line, and then a single dot is drawn above the line for each time the value occurs. This way, we end up with a number of dots above each value, dependent on the number of times that the value occurs
A simple example of a dot plot might be to consider the possible outcomes from rolling a single 6-sided dice 10 times and recording the results; { 1, 2, 5, 6, 2, 2, 4, 3, 5, 2 } and then drawing a dot plot:
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1 2 3 4 5 6
Here we see the possible values along the horizontal, and the number of dots above each represents the number of time that number was rolled. The horizontal line provides a convenient way of separating the dots from the values. Dot plots are useful for displaying categorical data (e.g. dice values), where there are a limited number of data points (say less than thirty).
Categories and Classes of data
Sometimes it is simple to categorise data according to value. For instance, the numbers on a rolled dice can be classified into the values D = {1, 2, 3, 4, 5, 6} and therefore lead to a simple representation in a dot plot. However, if we were to look at displaying the ages of people we meet on the street, it would be difficult to include them as categories: the number line might spread from 1 to 100 (possibly!). The Dot plot for this data would be very wide, and difficult to draw or read. A better idea would be to group the data according to some coarser categories (we call these ‘classes’) such as ages 1 to 10, 11 to 20, 21 to 30, etc.
These classes, or age groupings could then be displayed on a dot plot, e.g.:
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1-10 11-20 21-30 31-40 ...