Step by step
The question
Question: in a sample of 400, 120 say yes: find the 95% confidence interval for the true proportion.
1 Name the numbers
p prime is the part of the sample that says yes: x divided by n.
120 divided by 400 is 0.3.
q prime is the rest: 1 minus 0.3 is 0.7.
Careful!
p prime is the fraction 0.3, not the count 120.
Both counts, 120 and 280, are at least 5.
So the normal bell works here.
2 Find z*
95 percent of the bell is in the middle.
That leaves 5 percent: 2.5 percent in each end.
z* marks the cut between the middle and the ends.
Left of z* are the middle and one end: 0.95 plus 0.025 is 0.9750.
Now we look for 0.9750 inside the table.
0.9750 is in the table: row 1.9, column .06.
So z = 1.96.
3 Error bound
The error bound for a proportion: z* times the square root of p prime times q prime over n.
0.3 times 0.7 is 0.21.
Divided by 400: 0.000525.
The square root of that is about 0.0229.
Times 1.96: about 0.0449.
4 The interval
Take the error bound away from the middle: 0.3 minus 0.0449 is 0.2551.
And add it: 0.3 plus 0.0449 is 0.3449.
The 95 percent confidence interval is (0.2551, 0.3449).
5 Say it in words
In words: we are 95 percent confident that the true proportion is between 25.51% and 34.49%.
6 Check
Let's check: the middle of the interval should be 0.3.
0.2551 plus 0.3449, divided by 2, is 0.3.
Half the width should be the error bound: 0.3449 minus 0.2551, divided by 2, is 0.0449.
So it is right!
Let's look back.
First: name the numbers.
Then: find z*.
Then: error bound.
Then: the interval.
Then: say it in words.
Last: check.