Identifying Outliers
An outlier is typically defined as a value that is:
- More than 1.5 × IQR below Q1
- More than 1.5 × IQR above Q3
Lower fence: 20 - (1.5 × 20) = 20 - 30 = -10
Upper fence: 40 + (1.5 × 20) = 40 + 30 = 70
Outliers: Any values below -10 or above 70
Modified Box Plots
When outliers are present:
- Calculate outlier boundaries
- Draw whiskers to the last non-outlier values
- Plot outliers as separate points
This deep dive covers Special Cases and Outliers within Box Plots for GCSE Mathematics. Revise Box Plots in Statistics for GCSE Mathematics with 18 exam-style questions and 20 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 9 of 11 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Box Plots
On a box plot, what does the box (the rectangle) represent?
Two athletics clubs record the 100m sprint times (in seconds) for their members. The five-number summaries are shown below. Club A: Min = 11.2, Q1 = 12.4, Median = 13.1, Q3 = 14.2, Max = 16.5 Club B: Min = 12.0, Q1 = 13.5, Median = 14.8, Q3 = 16.1, Max = 17.3 Compare the distributions of sprint times for the two clubs. You must use the data to support your answer. (3 marks)