As always, thanks Bev! You are amazingly helpful for those statistically-challenged such as myself!

For the benefit of other Covers, and to be sure I've got this right, I'd like to maybe go step-by-step through a concrete, hypothetical, example.

Suppose specification is 20 +/- 2

**Step 1: Estimate Variance (standard deviation, SD)**
We could just "guesstimate" the variance. We are confident the process produces consistent outputs so the variance should be relatively small (SD<1).

Or, alternatively, we make a rough estimate by taking STDEV from, say, the first 10 units (

*aside: would this need to be justified?*)

STDEV(20,20,21,20,19,18,20,21,20,19) = 0.92

So, for this example,

SD = 0.92
**Step 2: What is the allowable tolerance (delta, d)**
This is taken from the specification (20 +/- 2). So the tolerance is:

d = 2
**Step 3: What is the sought confidence interval (t = 1 - alpha risk)?**
We will choose 95%. Looking up on the t-table, this is a value of:

t = 1.96
**Step 4: Calculate the sample size**
We use the formula for continuous data: n = [t*SD / d]^2

= [(1.96)*(0.92) / 2]^2 = 0.81

Rounding to the nearest integer, gives us

n = 1
Hence, according to the calculations above, only 1 sample is required in this example.

Do I have this correct?