Graphs (DP IB Psychology): Revision Note

A type of graphical display can be achieved using a bar graph

The data shown on the x-axis of a bar graph is discrete (not continuous)

• E.g., scores on a memory test; number of 'yes' answers ticked on a questionnaire

A bar graph uses categorical data which does not necessarily fall into any particular order

• If a researcher had conducted an experiment with three conditions, they could use a bar graph to display the means of each condition

Bar graphs do have gaps between each category on the x-axis (unlike histograms)

• The x-axis shows the categories/conditions

• The y-axis shows the score/percentage per category/condition

On a histogram, the x-axis represents the categories that have been measured, e.g.,

• the number of goals scored across one football season

• the number of marks in a psychology mock exam across one year group

On a histogram, the y-axis represents the frequencies of each category occurring, e.g.,

• the frequency of the number of two goals scored in one match

• the frequency of question 5 on the mock exam being awarded full marks

A histogram thus, shows continuous data

• Any category with zero frequency is represented by a space (a gap) in the chart

Histograms do not have gaps between the bars; the bars touch each other

A line graph shows how a quantity (continuous data) changes over time

• E.g.,  How the outside temperature changes during a week (shown below)

  • This could be of interest to psychologists who wish to investigate the effect of temperature on behaviour

Measurements of the quantity are taken at particular times

• Measurements should be taken at regular time intervals

• These are then plotted as points on a time series graph and joined together with straight lines

• The straight lines help to identify patterns and features in the data

Line graphs can show changes over short or long periods of time

• E.g., Changes to the memory scores of participants 30 seconds after being shown a list of words

• Or changes in memory scores of one group of people studied over several years

Sometimes a line graph may have more than one data set 

• E.g.,  one line for temperature and one line for number of arrests made on that day

Scatterplots are used to display the results of correlations

A scatterplot shows the point at which two separate pieces of data meet

Each co-variable can be presented along the x-axis or the y-axis

• E.g., a strong positive correlation will be shown regardless of which axis is chosen per co-variable

The arrangement of points on the scatterplot will indicate whether there is a positive correlation, a negative correlation or no correlation