Appendix 7: Interpretation of Boxplots Boffa Miskell Ltd | Waihi North Project: Freshwater Ecological Assessment | How to read a box plot/Introduction to box plots (from: https://www.wellbeingatschool.org.nz/information-sheet/understanding-and-interpreting-boxplots). Boxplots enable us to study the distributional characteristics of a group of scores as well as the level of the scores. To begin with, scores are sorted. Then four equal sized groups are made from the ordered scores. That is, 25% of all scores are placed in each group. The lines dividing the groups are called quartiles, and the groups are referred to as quartile groups. Usually we label these groups 1 to 4 starting at the bottom. Definitions • Median The median (middle quartile) marks the mid-point of the data and is shown by the line that divides the box into two parts. Half the scores are greater than or equal to this value and half are less. • Inter-quartile range The middle “box” represents the middle 50% of scores for the group. The range of scores from lower to upper quartile is referred to as the inter-quartile range. The middle 50% of scores fall within the inter-quartile range. • Upper quartile Seventy-five percent of the scores fall below the upper quartile. • Lower quartile Twenty-five percent of scores fall below the lower quartile. • Whiskers The upper and lower whiskers represent scores outside the middle 50%. Whiskers often (but not always) stretch over a wider range of scores than the middle quartile groups.
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