Data Visualization: Stem and Leaf Plots, Bar Charts, and Histograms, Slides of Business Administration

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Chapter 2

Presenting Data

Recap and Outline

• Frequency tables have limitations.
• Graphical methods can provide clearer picture.
• Use of computer packages.

Stem and Leaf Plots

Consider the following data

Stem Leaf

n = 8, stem unit = 10, leaf unit = 1.

Recovering the data

Observation = stem label × stem unit + leaf digit × leaf unit

Construct a stem and leaf plot for the following data on time (in

seconds) it takes to get through to an operator at a call centre:

Bar Charts

• Simple to produce.
• Easy to interpret.
• Applicable to categorical data and ungrouped discrete data.

Bar Charts

1. Decide what goes on each axis, by convention
   • Horizontal (x-axis) the variable being measured.
   • Vertical (y-axis) the frequency.
2. Find the maximum frequency.
3. Decide on an appropriate number scale for this axis.
4. Draw the axes.
5. Draw the bars.
   • All bars the same width.
   • All gaps between bars equal.

Bar Charts

 - Car Mode Frequency - Walk - Bike - Bus 

• Metro - Train
  • Total

Histograms

• Simple to produce.
• Easy to interpret.
• Applicable to grouped continuous data.

Histograms

For equal class interval histograms

1. Produce a grouped frequency table.
2. Find the maximum frequency.
3. Draw the vertical axis from zero to this maximum value.
4. Draw the horizontal axis and include the full range of classes.
5. Draw a bar for each class in the frequency table.

Example

Frequency table for the data on service times for a credit card call

centre is

Service time Frequency 175 ≤ time < 180 1 180 ≤ time < 185 3 185 ≤ time < 190 3 190 ≤ time < 195 6 195 ≤ time < 200 10 200 ≤ time < 205 12 205 ≤ time < 210 8 210 ≤ time < 215 3 215 ≤ time < 220 3 220 ≤ time < 225 1 Total 50