Step by step
The question
Question: samples of 81 are taken from numbers with mean 45 and SD 18: what is the probability that the sample mean is more than 48.5?
x̄, x bar, is the mean of one sample.
1 The sample means
The central limit theorem: the means of many samples make a bell.
Its middle is the mean, 45.
Its SD is smaller: σ divided by the square root of n.
The square root of 81 is 9.
18 divided by 9 is 2.
Careful!
The sample means have SD 2, not 18.
So we divide by 2.
2 Find z
z counts how many SDs the sample mean is from the mean: x̄ minus the mean, over the SD.
48.5 minus 45 is 3.5.
Divided by 2, that is 1.75.
3 Draw and shade
The bell: the mean, 45, in the middle.
48.5 is right of it.
We want the sample mean more than 48.5.
We shade that part.
4 Read the table
The z-table gives the area to the left of z.
Row 1.7, column .05.
For z = 1.75: the row 1.7 and the column .05 meet at 0.9599.
5 The area we want
The whole bell is 1.
The right side is 1 minus the left: 1 minus 0.9599 is 0.0401.
6 Check
Let's check another way.
The bell is the same on both sides.
So the area right of 1.75 is the area left of −1.75.
The table at −1.75 gives 0.0401.
The same!
So it is right!
Let's look back.
First: the sample means.
Then: find z.
Then: draw and shade.
Then: read the table.
Then: the area we want.
Last: check.