Interpreting results
Dual and compound bar charts
An ordinary bar chart shows one set of data. Put two sets side by side and you can compare them; stack them and you can see the total.
Two ways to show two sets
A plain bar chart shows ONE set of data. For two sets you have a choice: ā¢ A DUAL BAR CHART puts the two bars SIDE BY SIDE. ā¢ A COMPOUND BAR CHART STACKS them into a single bar. Both show the same numbers. They just make different things easy to see.
What a dual bar chart is good at
Side-by-side bars make COMPARING easy. With CDs and DVDs for three students: ā¢ Who has the most DVDs? Look for the tallest red bar. ā¢ How many more CDs than DVDs does Amber have? Read both bars and subtract: 8 ā 2 = 6. What it does NOT show at a glance is the total for each person.
What a compound bar chart is good at
Stacking the bars makes TOTALS easy. ā¢ Who has the most CDs and DVDs altogether? The tallest combined bar. ā¢ How many does Karl have in total? Read the top of his bar: 9. The trade-off: comparing the individual parts is harder, because they no longer start from the same line.
Choosing between them
Ask what the question is about. ā¢ Comparing the parts ā dual bar chart. ā¢ Comparing the totals ā compound bar chart. Either way, you need a KEY telling you which colour is which, a label on each axis, and a title. Without a key, neither chart means anything.
Worked examples
Who has the most DVDs?
- DVDs are the red bars.
- Find the tallest red bar.
Answer: Karl
Amber has 8 CDs and 2 DVDs ā how many more CDs?
- Read the blue bar: 8.
- Read the red bar: 2.
- 8 ā 2 = 6.
Answer: 6 more CDs
Who has the most CDs and DVDs in total?
- Use the compound chart.
- Find the tallest combined bar.
Answer: Wenkai
Karl's total from the compound chart
- Read the height of the whole stacked bar.
Answer: 9
Which chart compares two sets side by side?
- Side by side is the dual chart.
- Stacked is the compound chart.
Answer: A dual bar chart
What must every dual bar chart have?
- Two colours means you must say which is which.
- Plus axis labels and a title.
Answer: A key
Where you see this in real life
- Election coverage uses dual bars to compare parties in each region side by side.
- A budget is often shown as one compound bar, so you see both what each cost is and the total.
Key words
- Dual bar chart
- a chart with two sets of bars side by side
- Compound bar chart
- a chart with sets stacked into one bar
- Key
- the label saying which colour means what
- Axis
- a labelled line along the side or bottom of a chart
- Frequency
- how many of something there are