Box & Whisker Plots (DP IB Psychology): Revision Note

Box & whisker plots

• Box and whisker plots are used when researchers are interested in splitting data up into quartiles

• Often, data will contain extreme values

  • E.g., there are more people with an IQ of around 100 than there are people with an IQ of 130

    • If a researcher collects 50 IQ scores, 49 being from 'normal' people but one from a 'genius' (with an IQ of 130), then the 'genius' score would not fit in with the other data

• Using quartiles and drawing a box and whisker plot allows researchers to split the data so that they can see what is happening at the low, middle and high points

  • This allows them to also consider any possible extreme values

• Box and whisker plots include the following five values:

  • Lowest data value

  • Lower quartile

  • Median

  • Upper quartile

  • Highest data value

• On graph paper, box and whisker plots are drawn with the five values marked by short vertical lines

  • The middle three values then form a box with the median line inside

    • the median will not necessarily be in the middle of the box!

  • The box represents the interquartile range (middle 50% of the data)

  • The lowest data value and highest data value are joined to the box by horizontal lines:

    • these are the 'whiskers'

    • they represent the lowest 25% of the data and the highest 25% of the data

• The box and whisker plot below shows the number of items recalled by a class of four-year-old children when tested once a week over a month:

  • The median is 3 items

  • The upper quartile is 4.5 items

  • The lower quartile is 1 item

  • The whiskers show the maximum value of 5 items and the minimum value of 0 items