Sample Size

[Concillian]

Thank you, this is exactly the answer I was interested in and why I asked the question here.

I vaguely recall these terms of which you speak, from a previous lifetime  :P

Woohoo!  Only 8500 to go...
[right][snapback]96962[/snapback][/right]

Also, note that if you 'reverse' the situation in the calculator, you will see that the ~9600 value is for something with a near 50% rate (worst case). Something with a much lower or higher rate will require a lower sample size for a 95% CI = ±1%.

Because your sample size needed to know the rate within 1% is dependent on the actual drop percentage, you can't actually know the number you need for an accurate sampling until you get a few drops... fortunately you already have a decent idea with your current sampling.

Try putting reverse information in the other calculator on the page (entering your number of samplings + percentage + put in some very very large number for population) and you will see your CI. Then you can change the sample size and see how that adjusts the CI for your particular drop rate of interest. Likely your rate is fairly low (or it probably wouldn't be of interest) so you probably already have a decent approximation.

For example, if your drop rate is ~10% with 1000 drops then your confidence is ~±1.9%, to get it down to ± 1% you only need ~3500 samples, not the 9600 you need at 50%.

If you want even more information on sample sizes, you can look in statistics books about sample sizes for "1 proportion" tests. Books are usually geared towards comparing one sample to a known population, you can look at this specific case by setting the Beta error to .5 (setting it to 50% effectively removes it from the equation since it doesn't exist in this specific case) and the alpha error to 0.05 (5% error for a 95% confidence interval).