Step by step
The question
Question: a sample of 64 has mean 103, and the SD of all the numbers is 12: at the 5% level, is the true mean different from 100?
A test checks a claim with a sample.
We ask: if nothing new were going on, would a sample like ours be rare?
Alpha, 0.05, is the level, picked before we look at the data: a sample rarer than 5% counts as too rare.
1 Write the hypotheses
H₀ is the "nothing new" idea: it has the equal sign.
Here: the mean is 100.
Hₐ is what we want to show.
Here: the mean is different from 100.
Different from means too big or too small: both ends of the bell count.
Two tails.
2 The test number
We act as if H zero is true: the mean is 100.
How far is our sample mean from it?
First the SD of the sample means: σ over the square root of n.
The square root of 64 is 8.
12/8 is 1.5.
z is the sample mean minus 100, over 1.5.
103 minus 100 is 3.
Divided by 1.5: 2.
3 The p-value
The p-value is the chance of getting a sample at least this far out, if H zero is true.
On the bell of H zero: 100 in the middle, our sample at z = 2.
Two tails: we shade both ends, as far out as our sample or more.
The z-table gives the area to the left of z.
Row 2.0, column .00.
For z = 2: the row 2.0 and the column .00 meet at 0.9772.
Careful!
0.0228 is only one end.
Two tails: we double it.
One end is 1 minus 0.9772: 0.0228.
Two ends: 2 times that is 0.0456.
4 Decide
The p-value, 0.0456, is less than alpha, 0.05.
Our sample would be rare if H zero were true.
So we reject H zero.
5 Say it in words
In words: at the 5% level, there is enough evidence that the mean is different from 100.
6 Check
Let's check the other way, with the cut-off.
0.05 split into two ends gives the cut-offs plus and minus 1.96, from the table, as in the cut-offs lesson.
z is 2: past the cut-off.
The same decision.
So it is right!
Let's look back.
First: write the hypotheses.
Then: the test number.
Then: the p-value.
Then: decide.
Then: say it in words.
Last: check.