Step by step
The question
Question: in a sample of 120, 36 read every day: find the 90% 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.
36 divided by 120 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 36.
Both counts, 36 and 84, are at least 5.
So the normal bell works here.
2 Find z*
90 percent of the bell is in the middle.
That leaves 10 percent: 5 percent in each end.
z* marks the cut between the middle and the ends.
Left of z* are the middle and one end: 0.9 plus 0.05 is 0.9500.
Now we look for 0.9500 inside the table.
0.9500 is halfway between 0.9495 and 0.9505.
So z is halfway between 1.64 and 1.65: 1.645.
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 120: 0.00175.
The square root of that is about 0.0418.
Times 1.645: about 0.0688.
4 The interval
Take the error bound away from the middle: 0.3 minus 0.0688 is 0.2312.
And add it: 0.3 plus 0.0688 is 0.3688.
The 90 percent confidence interval is (0.2312, 0.3688).
5 Say it in words
In words: we are 90 percent confident that the true proportion is between 23.12% and 36.88%.
6 Check
Let's check: the middle of the interval should be 0.3.
0.2312 plus 0.3688, divided by 2, is 0.3.
Half the width should be the error bound: 0.3688 minus 0.2312, divided by 2, is 0.0688.
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.