History of Stem and Leaf Diagram

 

The Stem and Leaf plot was first used by John Tukey around 1977 in a study of volcanos.

John Tukey was a mathematician and statistician who is credited with first using the term software to describe the programs that run on computers.

Ability to organize data for analysis is important.

What's the difference between a frequency polygon and a frequency diagram

A plot of the cumulative frequency against the upper class boundary with the points joined by line segments. Any continuous cumulative frequency curve, including a cumulative frequency polygon, is called an ogive.

Frequency Density

Histograms

The following table shows the ages of 25 children on a school bus:
 Age  Frequency
5-10       6
11-15   15
16-17     4
> 17       0


If we are going to draw a histogram to represent the data, we first need to find the class boundaries. In this case they are 5, 11, 16 and 18. The class widths are therefore 6, 5 and 2.

The area of a histogram represents the frequency.

The areas of our bars should therefore be 6, 15 and 4.

Remember that in a bar chart the height of the bar represents the frequency. It is therefore correct to label the vertical axis 'frequency'.

However, as in a histogram, it is the area which represents the frequency.

It would therefore be incorrect to label the vertical axis 'frequency' and the label should be 'frequency density'.

Frequency density = frequency ÷ class width

A histogram is usually drawn when you have continuous data and the groups in the frequency table are of unequal size

To draw a histogram you will need to work out the frequency density. The frequency density can be calculated by using the following formula:

Frequency density = frequency ÷ class width

The class width is basically the width of the group.

The frequency density goes up the y-axis, and the area of each bar will represent the frequency.