24.2 Tables, bar charts, pie charts and pictograms — Learn

Prerequisites: tally charts, fractions, percentages and angle facts.

Objectives: organise categorical data in a frequency table; choose and construct a suitable chart; interpret charts for categories and ungrouped discrete values.

Tables and chart choice

A frequency is the number of times a category occurs. A frequency table lists each category and its frequency. Use a bar chart for separate categories: leave gaps between bars. A pictogram uses symbols, each worth a stated number. A pie chart shows parts of a whole; its sectors add to $360^\circ$.

For a pie chart, calculate each sector angle with

Example. In a survey of 40 pupils, 10 walk, 20 take a bus and 10 travel by car. Find the bus sector angle.

The bus fraction is $20/40=1/2$.

The angle is $1/2\times360^\circ=180^\circ$.

The bus sector is half the circle.

For ungrouped discrete numerical data, use a vertical line chart. Each line stands above a value, and its height shows that value's frequency.

Example. The data are $2, 5, 5, 7$. Find the frequency of each value.

The value 2 occurs once, 5 occurs twice, and 7 occurs once.

A vertical line chart has heights 1, 2 and 1 above 2, 5 and 7.

Predict: which category will have the most responses? Write your prediction down first.

Test: change the sample size and compare the counts.

Explain: why can the counts grow while the proportions stay the same? Write one sentence before reading on.

Watch out. A pictogram must state the value of one symbol. If one symbol means four pupils, a half-symbol means two pupils.

Progress check

For the travel survey, which chart would compare the numbers using each travel method? Give a reason.

You should now be able to

• make a frequency table from categorical data;
• choose a bar chart, pictogram, pie chart or vertical line chart for suitable data;
• calculate pie-chart angles and interpret a chart.