Higher only · cumulative frequency

Cumulative frequency.
Median and quartiles
from the curve.

a running total

the word itself —

Cumulative

adding up as you go

how many so far, up to each value

step 1 of 4

Running total

1. Keep adding: 6, then 6 + 14 = 20 …

2. The last one is the total: 40.

✓ It can never go down

time (min)	freq	cumulative
0 < t ≤ 10	6	6
10 < t ≤ 20	14	20
20 < t ≤ 30	12	32
30 < t ≤ 40	8	40

step 2 of 4

Plot at the upper end

1. Plot each total at the top of its class.

2. Start at (0, 0). Join with a smooth curve.

✓ (10, 6), (20, 20), (30, 32), (40, 40)

step 3 of 4

Median

1. Half of 40 is 20. Go across from 20.

2. Down to the axis: 20 minutes.

✓ Median = 20 minutes

step 4 of 4

Quartiles and IQR

1. Lower quartile: across from 10. Read 13.

2. Upper quartile: across from 30. Read 28.

✓ IQR = 28 − 13 = 15 minutes

cumulative frequency —

Plot

at the upper end of each class

Median

halfway up, then across and down

IQR

upper quartile − lower quartile

More than 25 minutes?
Read up, then take it from the total.

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