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Frequency tables, bar charts and histograms

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Data is easier to understand when it is organised and displayed. You need to construct and interpret tables and charts, and move between graphs and numbers.

Frequency tables

A frequency table shows how often each score or category occurs. Tally marks can be used, then counted.
Example: 12 participants rated their mood from 1 to 7: 3, 5, 4, 5, 6, 5, 4, 3, 5, 6, 7, 5.

Bar charts and histograms

Bar charts show data in separate categories (such as 'Condition A' and 'Condition B', or 'male' and 'female'). The bars have gaps between them. They are often used to compare the mean scores of conditions.
Histograms show continuous data (such as reaction times or test scores in ranges). The bars touch, because the data runs on without gaps. The height of each bar shows the frequency.
Both need a clear title and labelled axes: the x-axis (horizontal) and y-axis (vertical, usually frequency or mean score).

Reading graphs

To interpret a chart: read the axes carefully, check the units, then compare bars.
To translate between forms: a table can be drawn as a chart, and values can be read off a chart into a table.
Example: if a bar chart shows Condition A with a mean of 14 words and Condition B with a mean of 9, the difference is 14 - 9 = 5 words: Condition A recalled more.
Worked example

Mean words recalled: silence 15, music 11. Which chart should be used, and what does it show?

  1. The conditions are separate categories, so a bar chart with gaps is right.
  2. The silence bar would be taller: 15 - 11 = 4 more words recalled in silence.

Answer: A bar chart; it shows participants recalled 4 more words on average in silence.

Key idea

Frequency tables count how often scores occur. Bar charts show categories, with gaps between bars, and are used to compare means. Histograms show continuous data with touching bars. Always label axes and give a title.

The interactive lesson includes the diagrams for this topic.

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