Dot plots

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What is a dot plot

A dot chart or dot plot is a statistical chart consisting of data points plotted on a fairly simple scale, typically using filled in circles. There are two common, yet very different, versions of the dot chart


Example of a dot plot

Here is an example to show what a dot plot looks like and how to interpret it. Suppose 55 first graders are asked to pick their favourite number. Their choices can be summarized in a dot plot, as shown below.

Each dot represents one student, and the number of dots in a column represents the number of first graders who selected the number associated with that column. For example, 25 and 40 were the most popular (selected by 9 students), followed by 20 and 30(selected by 7 students). Selected by only 2 students, number 50 was the least popular.

In this example, note that the category is a qualitative variable; so it is not appropriate to talk about the symmetry or skewness of this dot plot. The dot plot in the next section uses a quantitative variable, so we will illustrate skewness and symmetry of dot plots in the next section

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how to make a dot plot

  • 1.Find the minimum and maximum value in your data set. The minimum value is the smallest value in the set, the maximum value is the largest.

    2.Draw a number line from the minimum value to the maximum value, incrementing evenly, based on your set of data. For instance, if your data only includes integers between 10 and 14, the number line would only need to list 10, 11, 12, 13, 14. Increment in decimals if your data includes decimals.

  • 3.Place a dot above the number on the number line for each number in your data set. If there is already a dot above the number, add an additional dot above the existing one. When you finish, there should be a dot on the graph for each piece of data in the data set.

    pros and cons about dot plots

    Advantages

    • easy to make
    • show each individual data point

    Disadvantages

    • can be time consuming with lts of data points to make
    • have to count to get exact total
    • Fractions of units hard to display