Sample size: 95% confidence, sd 15, error bound 2 Step by Step

Step by step

The question

Question: the SD of all the numbers is 15: how big a sample do we need to be 95% confident with an error bound of 2?

n? 95% confidence, σ = 15, EBM = 2

The error bound is how far the sample mean may be from the true mean.

EBM: how far the sample mean may be from the true mean

1 Find z*

95 percent of the bell is in the middle.

That leaves 5 percent: 2.5 percent in each end.

95% in the middle: 5% left over, 2.5% 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.

left of z* = 0.95 + 0.025 = 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.

z* = 1.96

2 The formula

The error bound is z* times σ over the square root of n.

Solved for n, it gives this formula.

from EBM = z* × σ ÷ √n, solved for n

n is z* squared times σ squared, over the error bound squared.

n = z*² × σ²EBM²

3 Put the numbers in

1.96 squared is 3.8416.

15 squared is 225.

2 squared is 4.

3.8416 times 225 is 864.36.

Divided by 4: 216.09.

n = 1.96² × 15²2² = 3.8416 × 2254 = 864.364 = 216.09

4 Round up

A sample size must be a whole number.

We always round up: a smaller sample would miss the goal.

always round UP: a bigger sample is safe

216.09 rounds up to 217.

n = 217

Careful!

Rounding down to 216 gives an error bound a bit too big.

Always round up.

Careful: not n = 216 ✗

5 Check

Let's check: with 217 people the error bound is about 1.9958.

That is not more than 2.

Good.

n = 217: EBM ≈ 1.9958 ≤ 2

With one person less, 216, it is about 2.0004: too big.

So 217 is the smallest that works.

So it is right!

n = 216: EBM ≈ 2.0004 > 2

Let's look back.

First: find z*.

Then: the formula.

Then: put the numbers in.

Then: round up.

Last: check.

Answer