Step by step
The question
Question: in group 1, 70 of 200 say yes, and in group 2, 50 of 200: at the 5% level, is the first proportion more than the second?
1 Write the hypotheses
H₀ is the "nothing new" idea: it has the equal sign.
Here: the two proportions are the same, so the difference is 0.
Hₐ is what we want to show.
Here: the first proportion is more than the second.
More than: one tail, the right end.
2 The two parts
Group 1: 70/200 is 0.35.
Group 2: 50/200 is 0.25.
3 Pool the groups
If H zero is true, the two groups are the same.
So we put them together: the pooled proportion.
All the yeses over everyone: 120/400 is 0.3.
Careful!
We pool the counts, not the two proportions.
And q c is 1 minus 0.3: 0.7.
4 Standard error
The standard error, the SD of the difference: the square root of p c times q c, times 1/n 1 plus 1/n 2.
0.3 times 0.7 is 0.21.
1/200 plus 1/200 is 0.01.
Multiply, then the square root: about 0.0458.
5 The test number
z is the difference over the SE.
0.35 minus 0.25 is 0.1.
Divided by 0.0458: about 2.18.
6 The p-value
The p-value is the chance of getting a sample this far out, if H zero is true.
On the bell of H zero: 0 in the middle, our sample at z = 2.18.
We shade the right end, from our z up.
The z-table gives the area to the left of z.
Row 2.1, column .08.
For z = 2.18: the row 2.1 and the column .08 meet at 0.9854.
The right end is 1 minus 0.9854: 0.0146.
7 Decide
The p-value, 0.0146, is less than alpha, 0.05.
Our sample would be rare if H zero were true.
So we reject H zero.
8 Say it in words
In words: at the 5% level, there is enough evidence that the first proportion is more than the second.
9 Check
Let's check the other way, with the cut-off.
0.05 in the right end gives the cut-off 1.645.
z is 2.18: past the cut-off.
The same decision.
So it is right!
Let's look back.
First: write the hypotheses.
Then: the two parts.
Then: pool the groups.
Then: standard error.
Then: the test number.
Then: the p-value.
Then: decide.
Then: say it in words.
Last: check.