Digits instead of crosses
Stacked dot plots group the values into classes, so some detailed information about the values is lost.
A clever way to retain some of this lost detail replaces each cross with a digit (0 to 9) that shows information about the position of the value within its stack.
Nursing home residents
This example does not involve assessment, but illustrates the idea of using digits instead of crosses better than most data sets involving marks.
The stacked dot plot below shows the number per 500 population aged 65 or more who are nursing home residents in each state of the USA.
Drag with the mouse over the crosses to discover the names of the states.
The crosses only allow you to read off the 'units' digits of the values — Michigan (18.0) and Wyoming (18.9) are on the same stack of crosses. Select Digits from the pop-up menu. The crosses are replaced by the 'tenths' digits of the values.
Stem and leaf plots
A stem and leaf plot is basically the same — a stacked dot plot with digits instead of crosses — but the 'axis' is drawn vertically and is laid out differently with a stem value showing the most significant digits for each stack. The digits representing the values on the stacks are called leaves in the terminology of stem and leaf plots.
Stem and leaf plots are usually drawn with fewer stacks of values (classes) than stacked dot plots. About 10 stacks is usually enough.
The format is most easily understood with an example.
Maths test marks
The stem and leaf plot below shows the maths marks from the previous page.
Click on any leaf in the plot (a black digit). The corresponding data value is shown above the plot.
Observe that the stem is the 'tens' digit of the value and the leaf is its 'units' digit.