Step by step
The question
Question: boys: 22 sport, 18 music, 20 art; girls: 18 sport, 22 music, 20 art: at the 5% level, is the answer linked to the group?
1 Write the hypotheses
H₀ is the "nothing new" idea.
Here: the group and the answer are independent.
Hₐ is what we want to show.
Here: the group and the answer are not independent.
Independent means no link: the answer does not depend on the group.
2 Totals
The counts in a table.
Each row's total: boys, 60; girls, 60.
Each column's total: sport, 40; music, 40; art, 40.
And everyone: 120.
3 Expected counts
If there is no link, each cell expects its row total times its column total, over the grand total.
Boys, sport: 60 times 40, over 120, is 20.
Boys, music: 60 times 40, over 120, is 20.
Boys, art: 60 times 40, over 120, is 20.
Girls, sport: 60 times 40, over 120, is 20.
Girls, music: 60 times 40, over 120, is 20.
Girls, art: 60 times 40, over 120, is 20.
4 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.
22 minus 20 is 2.
Squared, 4.
Over 20: 0.2.
18 minus 20 is −2.
Squared, 4.
Over 20: 0.2.
20 minus 20 is 0.
Squared, 0.
Over 20: 0.
18 minus 20 is −2.
Squared, 4.
Over 20: 0.2.
22 minus 20 is 2.
Squared, 4.
Over 20: 0.2.
20 minus 20 is 0.
Squared, 0.
Over 20: 0.
5 Add them
0.2 plus 0.2 plus 0 plus 0.2 plus 0.2 plus 0 is 0.8.
6 The cut-off
df is rows minus 1, times columns minus 1: 1 times 2 is 2.
Careful!
Not the number of cells minus 1.
Rows minus 1, times columns minus 1: 2.
Chi-square is always a right-tail test: big means far from what we expected.
Row 2, column 0.05.
Row 2, column .05: the cut-off is 5.991.
7 Decide
The chi-square curve for df 2.
Past the cut-off, 5.991, is the reject part.
0.8 is not past the cut-off.
So we do not reject H zero.
8 Say it in words
In words: at the 5% level, there is not enough evidence that the group and the answer are not independent.
This does not prove H zero is true.
We just cannot rule it out.
9 Check
Let's check: the expected counts give the same row totals.
Boys: 20 plus 20 plus 20 is 60.
Girls: 20 plus 20 plus 20 is 60.
So it is right!
Let's look back.
First: write the hypotheses.
Then: totals.
Then: expected counts.
Then: each part.
Then: add them.
Then: the cut-off.
Then: decide.
Then: say it in words.
Last: check.