Presentation and display of quantitative data: graphs, tables,…

The idea

A table in a report is not a list of raw scores; it is a summary, giving the descriptive statistics for each condition under a title that names the variables and the units. Bar charts display data in categories or conditions, and their bars carry gaps because there is no continuous scale along the bottom, while the vertical axis shows frequency or a mean. Histograms display continuous data, so the bars touch, the horizontal axis is divided into equal intervals and no interval is skipped. A scattergram displays a correlation: one co-variable on each axis, one dot per participant, and the shape of the cloud tells you direction and strength. Label both axes and give a title every time. Watch for a vertical axis that does not begin at zero, which makes a trivial difference look dramatic.

What earns the marks

• Choosing the right display for the data: bar chart for categories or conditions, histogram for continuous data grouped into intervals, scattergram for a correlation.
• Gaps between the bars of a bar chart because the categories are discrete, and touching bars on a histogram because the underlying scale is continuous.
• A summary table presents descriptive statistics rather than raw data, and is read with a sentence saying what it shows.
• Reading a scattergram: an upward pattern is a positive correlation, a downward pattern negative, and a formless scatter of points a zero correlation.
• Evaluation: a graph can mislead, for instance where the vertical axis does not start at zero, so an apparently large difference may be trivial.

Common mistakes and how to put them right

• Drawing a bar chart with touching bars, or a histogram with gaps, which forfeits the mark that distinguishes the two.

• Plotting two means as a bar chart when the stem describes a correlation, which needs a scattergram.

• Leaving the axes unlabelled, which is the most commonly lost mark on any drawing item.

• Copying every participant's raw score into what the question called a summary table.

Exam tips

• Read the level of measurement in the stem before choosing the graph; categories mean a bar chart whatever the study is about.
• 'Sketch' still requires labelled axes and a title, but the plotting need not be to scale.
• When asked what a graph shows, describe the pattern using the stem's variables rather than saying one bar is taller than another.