Stem and Leaf Plots

Stem and leaf plots are very easy to create by hand and are used for a one-pass ``quick and dirty'' look at the data. We will use stem and leaf plots for the following:

• Primarily for continuous data:
  1. The leading digit(s) give the STEM.
  2. The trailing digit(s) give the LEAF.
• Mechanics
  1. First form groups or categories.
  2. Write out the stems.
  3. Add the leaves.
  Usually there is no need to sort the leaves, although computer packages typically do.

EXAMPLE: The first exam scores for 25 students enrolled in an introductory statistics course are given below.

A stem and leaf plot for these data is

• What it shows us:
  1. The center of the distribution.
  2. The overall shape of the distribution,
     • If the portions on each side of the center value are mirror images of each other.
     • Left If the left tail (lower values) is much longer than the right tail (higher values).
     • Right If the right tail (higher values) is much longer than the left tail (lower values).
  3. Marked deviations from the overall shape of the distribution.
     • Individual observations that fall well outside the overall pattern of the data.
     • gaps in the distribution
• Advantages:
  1. Very easy to compute, can be done by hand.
• Disadvantages:
  1. Only good to get a percentage of observations in a range.
  2. Picture itself not very useful for comparing more than one population.