Cumulative Frequency — The Accumulator
Construct · Plot · Read · Bridge Strategy
▶ The Accumulator — Visual Toy
◈ Logic Track
What is Cumulative Frequency?
"Cumulative" means running total. We add each frequency to the sum of all previous frequencies. This tells us: "How many values are below this upper boundary?"
📐 The Rule
Plot at the UPPER boundary of each class interval — never the midpoint.
⚡ Dynamic Solver — Cambridge Mark Scheme Steps
Click "▶ Start" above to begin building the table and see live step-by-step working.
Phase 1 · Anchor
Build Confidence
Click through each class interval. Watch the running total grow and each point plot at the upper boundary.
Phase 2 · The Twist
The S-Curve Forms
Notice the curve steepens then flattens. The steepest section reveals the modal class (most frequent group).
Phase 3 · Killer
Exam Technique
To estimate the median: draw a horizontal line at n/2 = 26, then read off the x-value. Draw vertical construction lines clearly.
⚠ Examiner Trap Vault — Cumulative Frequency
🚨 Plotting at Midpoint
❌ Plot (5, 5), (15, 17)…
✓ Plot (10, 5), (20, 17)…
🚨 Forgetting the Origin
❌ Start curve from first data point
✓ Start from (0, 0) — zero values below the lowest boundary
🚨 Drawing Straight Lines
❌ Joining points with a ruler
✓ Draw a smooth freehand S-curve through all points
🚨 Reading Median Incorrectly
❌ Use n/2 = 25 (not 26 for n=50)
✓ Use n/2 for even n. Draw construction lines clearly for method marks.
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