Foundation and Higher · frequency trees

Frequency trees.
Sort the people,
then read it off.

every branch pair
adds up to its parent

a frequency tree holds —

Numbers

how many people, not probabilities

each split must add back to the total

step 1 of 4

Start with the total

1. 100 students. 60 are in Year 10.

2. The rest are Year 11: 100 − 60 = 40.

✓ 60 + 40 = 100

step 2 of 4

Split again

1. 45 Year 10s walk: 60 − 45 = 15 do not.

2. 22 Year 11s walk: 40 − 22 = 18 do not.

✓ Each pair adds to the number before

step 3 of 4

Read a probability

1. How many walk? 45 + 22 = 67.

2. Out of all 100 students.

✓ P(walks) = 67100

step 4 of 4

Given that…

1. A Year 11 is chosen: only those 40 count.

2. 22 of them walk.

✓ P(walks) = 2240 = 1120

frequency trees —

Fill in

each pair adds to its parent

Probability

the count you want ÷ the total

“Given that”

the total becomes that branch

Frequencies are whole numbers.
Probabilities go on a different tree.

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