Stage 12 · Data handling
Compound bar charts
Read the key, then the scale.
How to do it
Method
Read the key, then the scale
Each line 2: 8 × 2 = 16, 6 × 2 = 12, 16 − 12 = 4
Another way
Count lines first
8 − 6 = 2, 2 × 2 = 4
In full
A compound bar chart puts two sets of data side by side: two bars for each item. First read the key, which colour is which set; then see what each grid line stands for (not always 1). Each line 2, the bar reaching the 8th line: 8 × 2 = 16. Comparing an item's two bars: 4A has 16 girls and 12 boys, 16 − 12 = 4, so 4 more girls; one set across two items, such as the boys of 4A and 4C, is taken away the same way. For a set's total, add its bars one by one. Quicker: count grid lines, and multiply only at the end: girls 8 lines, boys 6, 8 − 6 = 2, 2 lines apart, 2 × 2 = 4 — the same answer.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
Adults in Feb: ? people
Show the answer and working
Answer: Adults in Feb: 4000 people
Key: the left bars are children, the right adults; each grid line stands for 1000. Feb, adults: line 4, 4 × 1000 = 4000
Example 2
Children: Mar − Jan = ?
Show the answer and working
Answer: Children: Mar − Jan = 3000
Key: the left bars are children, the right adults; each grid line stands for 1000. Children: Mar: 4 × 1000 = 4000; Jan: 1 × 1000 = 1000; 4000 − 1000 = 3000
Example 3
Most in one class: ? pupils
Show the answer and working
Answer: Most in one class: 26 pupils
Each grid line stands for 2. Count each class's two bars in grid lines: 4A 1 + 5 = 6; 4B 1 + 1 = 2; 4C 7 + 6 = 13; 4D 4 + 4 = 8. The most is 4C, 13 lines, 13 × 2 = 26
Example 4
5C in all: ? books
Show the answer and working
Answer: 5C in all: 400 books
Key: the left bars are Chinese, the right English; each grid line stands for 50. 5C: Chinese: line 2, 2 × 50 = 100; English: line 6, 6 × 50 = 300; 100 + 300 = 400
Practise compound bar charts in the game →Stage 12 · Data handling · Free to start, unlimited questions, no download