Choosing the right average: mean, median or mode. Year 10 Foundation OCR GCSE Unit 7 Lesson 9.

Pupils identify outliers and learn why the median is unaffected by them — while the mean drags upward or downward. The decision table (categorical → outlier check → mean as default) becomes the algorithm they apply to every dataset.

What’s included:

• I Do (sleep data, no outlier) and We Do (goals per match, clear outlier) side-by-side contrast
• Four independent practice tasks — one categorical, one with tight clustering, two with outliers to catch
• Three-misconception Spot the Mistake — confusing “no mode” with “no average”, missing the outlier, misapplying the mean
• Going Deeper: a dataset where mean > median > mode (positive skew) — pupils diagnose the shape
• Exit Ticket trap: all three averages happen to equal 5, but median is the conceptually correct answer because of outlier-resistance (even when the mean got lucky)

Teacher key (5 pages):

• Full answers and calculations across all tasks
• Diagnostic-for-marking: why pupils choose mode when median is intended (both are valid; median directly addresses the outlier)
• OCR mark scheme alignment
• Conceptual trap notes for the Exit Ticket — full marks only for identifying the outlier AND choosing median because of design principle, not numerics

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