IGCSE Statistics Exam Technique
Statistics and probability is the topic where careful reading separates A* students from B students. The numbers in a statistics question are just arithmetic — the challenge is understanding what the question is actually asking.
Mean from Frequency Tables
Always add a 'midpoint × frequency' column to the table before calculating. The mean is Σ(fx) ÷ Σf, not Σf ÷ n. Students who rush this step repeatedly divide by the wrong denominator.
Tips
- Use the midpoint of each class interval, not the boundary
- Add a running total column for cumulative frequency questions
- If the table gives raw frequencies, always check they sum to the total given in the question
Cumulative Frequency and Box Plots
Plot cumulative frequency at the upper class boundary, not the midpoint. The median is at n/2, not (n+1)/2 — read horizontally to the curve, then down to the x-axis.
Tips
- Mark n/2, n/4 and 3n/4 on the y-axis before reading off values
- Interquartile range = UQ − LQ, never UQ + LQ
- For box plots, the whiskers extend to the actual minimum and maximum, not estimated values
Histograms
Histograms use frequency density, not frequency, on the y-axis. Frequency density = frequency ÷ class width. This catches out students who plot frequency directly and lose all histogram marks.
Tips
- Write 'frequency density = frequency ÷ class width' at the top of your working
- If the question asks you to find a frequency from a histogram, multiply frequency density by class width
- Check your y-axis label before drawing — examiners look for this
Probability
For combined events, draw a tree diagram or sample space grid first. Never calculate P(A and B) by adding — multiply for independent events, use conditional fractions for dependent events.
Tips
- Label all branches with fractions before calculating
- Check that all branches from one node sum to 1
- For 'without replacement' questions, reduce the denominator on the second branch
Key Takeaways
- Plot cumulative frequency at upper class boundaries, not midpoints
- Histograms always use frequency density — never plot raw frequencies
- Always draw a tree diagram before calculating combined probabilities
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What is the most common statistics mistake in IGCSE 0580?
Plotting cumulative frequency at midpoints instead of upper class boundaries. This shifts the entire curve and makes median, quartiles, and percentile readings wrong.
Can I use a calculator for statistics questions?
Yes — statistics questions appear in Papers 2 and 4 (calculator allowed). Make sure your calculator can handle statistical mode for mean and standard deviation checks.
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