Lesson Plan

• Describe the purpose of measures of dispersion and differentiate between range, IQR, and standard deviation.
• Calculate the range, inter‑quartile range, and sample standard deviation for a given data set using manual methods.
• Interpret the results of each dispersion measure to assess data spread and identify outliers.
• Apply the appropriate measure of dispersion to solve typical IGCSE statistics problems.

• Whiteboard and markers
• Projector with slides showing formulas and box‑plot diagram
• Calculator (teacher demo) and student worksheets with data sets
• Rulers for constructing box‑plots
• Printed handout of step‑by‑step calculation tables

Begin with a quick real‑world question: “How can we tell if two classes performed similarly even if their averages are close?” Review that mean, median and mode locate the centre of data, then state that today’s success criteria are to compute and interpret three key dispersion measures.

1. Do‑now (5’) – Students calculate the range of a short data list on the board to recall the formula.
2. Mini‑lecture (10’) – Introduce IQR, demonstrate ordering data, finding Q₁ and Q₃, and sketch a box‑plot.
3. Guided practice (12’) – Whole‑class work through the worked example, filling a table for standard deviation step‑by‑step.
4. Pair activity (10’) – Learners use a new data set to compute range, IQR and sample standard deviation, checking answers with a teacher‑provided answer key.
5. Check for understanding (8’) – Quick exit‑ticket: write one situation where IQR is preferred over range.
6. Summary (5’) – Recap key formulas and when each measure is most useful.

Summarise that range gives a quick spread, IQR shows the middle 50 % robustly, and standard deviation measures average deviation from the mean. Ask students to complete a short homework worksheet calculating dispersion for a set of test scores. Collect exit tickets to gauge understanding.