Quantitative data charts, also called grouped data, can be viewed using a histogram or a polygon.
How to graph quantitative data
- Place the classes on the horizontal axis
- You can place either the frequency distribution, the relative frequency, or the percentage on the vertical axis.
In the lesson on creating a frequency distribution, we ended with this table showing the frequency distribution, relative frequency, and percentage distribution.
Let’s use a histogram and a polygon to display the data. There is little difference between the two charts except that the histogram uses rectangles and the polygon uses points.
In both cases we will use the frequency distribution to build the graphs.
First, put the classes on the horizontal axis. Second, put the frequencies on the vertical axis.
Finally, draw a rectangle for each class so that the height of the rectangle corresponds to the frequency of that class.
Notice that all the rectangles are adjacent and, unlike a bar graph, have no gaps between them.
That histogram above is called a Frequency histogram.
If you had used relative frequency to build the histogram, you would call the chart a Relative Frequency Histogram.
If you had used the percentage to build the histogram, you would call the chart a percentage histogram.
Simply use points instead of rectangles to draw a polygon.
Place a point just above the midpoint of each class.
The height of the point should correspond to the frequency of this class.
Create 2 more classes. The first is created before 9-18 and the second after 59-68. Both classes have a frequency of 0.
Connect all points with straight lines.
Notice that this time we didn’t cut off the x-axis as it uses all of the values on the x-axis.
The above diagram is called a frequency polygon or simply a polygon.
If you had used relative frequency to create the polygon, you would name the graph a Relative frequency polygon.
If you had used the percentage to build the histogram, you would call the chart a Percent polygon.