Box & Whisker Diagrams (DP IB Analysis & Approaches (AA)): Revision Note

Box plots

What is a box plot (box and whisker diagram)?

• A box plot (or box and whisker diagram) is a graph that clearly shows key statistics from a data set

  • It shows the median, quartiles, minimum and maximum values and outliers

  • It does not show any other individual data items

• The middle 50% of the data will be represented by the box section of the graph and the lower and upper 25% of the data will be represented by each of the whiskers

• Any outliers are represented with a cross on the outside of the whiskers

  • If there is an outlier then the whisker will end at the value before the outlier

• Only one axis is used when graphing a box plot

  • It is still important to make sure the axis has a clear, even scale and is labelled with units

What are box plots useful for?

• Box plots can clearly show the shape of the distribution

  • If a box plot is symmetrical about the median then the data could be normally distributed

• Box plots are often used for comparing two sets of data

  • Two box plots will be drawn next to each other using the same axis

  • They are useful for comparing data because it is easy to see the main shape of the distribution of the data from a box plot

    • You can easily compare the medians and interquartile ranges