Step by step
The question
Question: a die is rolled 300 times, and the faces 1 to 6 come up 42, 55, 38, 61, 50, 54 times: at the 5% level, is the die fair?
1 Write the hypotheses
H₀ is the "nothing new" idea.
Here: the die is fair.
Hₐ is what we want to show.
Here: the die is not fair.
2 Expected counts
If H zero is true, each cell expects n times its part.
Each face: 300 times 1/6 is 50.
Every expected count is at least 5, so the test works.
3 Each part
Chi-square adds up, for each cell, the observed minus the expected, squared, over the expected.
A table: each cell, what we saw, O, and what we expected, E.
42 minus 50 is −8.
Squared, 64.
Over 50: 1.28.
55 minus 50 is 5.
Squared, 25.
Over 50: 0.5.
38 minus 50 is −12.
Squared, 144.
Over 50: 2.88.
61 minus 50 is 11.
Squared, 121.
Over 50: 2.42.
50 minus 50 is 0.
Squared, 0.
Over 50: 0.
54 minus 50 is 4.
Squared, 16.
Over 50: 0.32.
Careful!
We always divide by the expected count, not the observed one.
4 Add them
1.28 plus 0.5 plus 2.88 plus 2.42 plus 0 plus 0.32 is 7.4.
5 The cut-off
df is the number of cells minus 1: 6 minus 1 is 5.
Chi-square is always a right-tail test: big means far from what we expected.
Row 5, column 0.05.
Row 5, column .05: the cut-off is 11.070.
6 Decide
The chi-square curve for df 5.
Past the cut-off, 11.070, is the reject part.
7.4 is not past the cut-off.
So we do not reject H zero.
7 Say it in words
In words: at the 5% level, there is not enough evidence that the die is not fair.
This does not prove H zero is true.
We just cannot rule it out.
8 Check
Let's check: the expected counts add up to 300, like the observed ones.
And the differences O minus E add up to 0.
So it is right!
Let's look back.
First: write the hypotheses.
Then: expected counts.
Then: each part.
Then: add them.
Then: the cut-off.
Then: decide.
Then: say it in words.
Last: check.