🧪 Conditions
Bars can show the conditions of an experiment, such as the mean number of darts on target after 4 hours of sleep and after 8 hours of sleep.
Dealing with Data » Bar Charts
What you'll learn this session
Study time: 30 minutes
Cambridge spec: 7.9.7, 7.9.8
Exam: Paper 1 and Paper 2
- What a bar chart is used for in psychology
- The format of a bar chart: axes, labels, placement of bars and units
- How to read a bar chart you have never seen before
- How to sketch a bar chart and spot mistakes in one
A table is good for storing numbers. A bar chart is good for seeing them. A quick look at the bars shows which group scored highest and which scored lowest. Bar charts can appear in both Paper 1 and Paper 2 (Sections 7.5 to 7.9 are tested in both), and they are always about a study you have not met before.
🔒 Unlock Full Course Content
Sign up to access the complete lesson and track your progress!
Unlock This CourseWhat a bar chart is used for
Key terms:
- Bar chart: a chart that uses separate bars to show and compare values for different categories or conditions.
- Axis: one of the two lines that a chart is built around. The x-axis goes across the bottom. The y-axis goes up the side.
- Label: words on an axis (and a title for the chart) that say what is being shown.
In psychology, a bar chart is mainly used to compare categories or conditions. A very common choice is to show the mean score in each condition of an experiment, using the means you worked out in the last lesson, Mean, Median, Mode and Range. The taller the bar, the higher the value.
👥 Categories
Bars can show groups of people, such as the mean number of times children smiled at a stranger at age 5, age 8 and age 11.
The format of a bar chart
Every bar chart has the same parts. Learn them as a checklist.
📏 Axes
The x-axis (across the bottom) shows the conditions or categories, one for each bar. The y-axis (up the side) shows the measured score, such as the mean. The y-axis has a scale of equal steps, such as 0, 2, 4, 6, 8, and it should start at 0.
🏷 Labels
Each axis needs a label saying what it shows. The x-axis label might be "Hours of sleep". The y-axis label might be "Mean number of darts on target". The chart also needs a title that says what the whole chart shows.
📊 Placement of bars
The bars are separate, with gaps between them. They are all the same width and stand on the x-axis. Their height shows the score.
⏱ Units
Where the score is measured in something, the unit goes in the axis label, usually in brackets, such as "Mean time to finish (seconds)". A count, such as a number of darts, has no unit, so the label alone is enough.
Learn this
Conditions or categories go on the x-axis. The measured score goes on the y-axis. Bars have gaps between them. Both axes and the whole chart are labelled, with units where appropriate.
Reading a bar chart
Here is a new scenario. A researcher asked three groups of pupils to learn the same spelling list using different methods: reading it silently, writing it out, or saying it aloud. Each pupil then took a spelling test out of 20. The bar chart has three separate bars, and the same results are in this table.
| Revision method (x-axis) | Mean spelling score out of 20 (y-axis) |
|---|---|
| Reading silently | 11 |
| Writing it out | 15 |
| Saying it aloud | 13 |
The bars reach 11, 15 and 13 on a y-axis that goes from 0 to 20. To read a bar chart, follow these steps.
- Read the title and both axis labels to see what is being shown, and look for units.
- Check the scale on the y-axis. Work out what each small step is worth.
- Find the bar you need, then go across from the top of the bar to the y-axis to read its value.
- To compare, look at the bar heights. The tallest bar is the highest value.
Worked example
Using the spelling results, which method gave the highest mean score, and how much higher was it than reading silently? [2 marks]
Writing it out gave the highest mean score (1). It was 4 marks higher, because 15 − 11 = 4 (1).
Sketching a bar chart
You may be asked to sketch a bar chart from a table or from written information. A sketch does not have to be perfect, but it must have the right format. Use this order:
- Draw the two axes.
- Choose an even scale for the y-axis that starts at 0 and goes a little above the highest value.
- Put one condition or category on the x-axis for each bar, and leave gaps between the bars.
- Draw each bar to the right height.
- Label both axes, add a title, and add a unit where appropriate.
Try it with this table from a study on sleep and aiming. Pupils threw 10 darts after 4 hours of sleep or after 8 hours of sleep.
| Hours of sleep | Mean number of darts on target (out of 10) |
|---|---|
| 4 hours | 3 |
| 8 hours | 6 |
Your sketch has two bars with a gap between them. The x-axis is labelled "Hours of sleep", with the bars named 4 hours and 8 hours. The y-axis is labelled "Mean number of darts on target", with a scale from 0 to 10. The 4-hour bar reaches 3 and the 8-hour bar reaches 6.
Spotting mistakes in a bar chart
A question may describe a poor chart and ask what is wrong with it. Use the format checklist. Here are the mistakes examiners expect you to find.
- Bars touching. Bars should be separate, with gaps.
- Missing labels. One or both axes have no label, or there is no title.
- Wrong axes. The mean score is on the x-axis and the conditions are on the y-axis. They should be the other way round.
- Missing unit. A time is shown but the label does not say seconds or minutes.
- Uneven scale. The y-axis jumps 0, 2, 4, 10, 12, so the bars give a false picture.
- Bars not matching the data. A bar is drawn to a different height than the value in the table.
Worked example
A bar chart of children's mean number of smiles has no label on the y-axis, and the bars for age 5, age 8 and age 11 are touching. Give two mistakes in the chart. [2 marks]
The y-axis has no label, so we do not know what the heights show (1). The bars are touching, but they should be separate with gaps between them (1).
Common mistakes
Common mistakes
- Putting the score on the x-axis and the conditions on the y-axis. The conditions or categories go along the bottom.
- Drawing bars that touch. Leave gaps between them.
- Forgetting the labels, the title or the unit. Each can be a mark in a sketch question.
- Reading the top of a bar wrongly because you did not check the scale. Work out what each step on the y-axis is worth.
- Using a scale that does not start at 0 or has uneven steps.
- Saying a bar is "bigger" when you mean "taller" or "higher". Use the numbers, such as "the mean was 4 higher".
Exam-style question
A researcher timed how long pupils took to finish a puzzle, in a quiet room or with a loud noise. The mean time in the quiet room was 40 seconds. The mean time with the loud noise was 55 seconds.
(a) The researcher draws a bar chart of the results. State what should be on the x-axis. [1 mark]
(b) Describe how the researcher should show the unit on the y-axis. [1 mark]
(c) The researcher's chart has bars that touch and no title. Give two things that should be changed. [2 marks]
Model answer
(a) The two conditions, quiet room and loud noise (1).
(b) Label the y-axis with the unit in brackets, such as "Mean time to finish the puzzle (seconds)" (1).
(c) The bars should be separate with gaps between them (1). The chart needs a title saying what it shows (1).
Exam tip
For any question about a bar chart, name the exact thing in the scenario. Write "the mean time to finish the puzzle (seconds)" rather than just "the score".