This got me thinking. How do you know how many samples you need to measure to get a reliable standard deviation? For example, say I'm manufacturing cereal. I want to put 300g +/- 5 grams in every box. To determine if I'm doing that, I would used the one sample t-test power and sample size calculation. If I didn't have a database of historical weights with a large number of samples which I could use to calculate the standard deviation, how many samples would I need to weigh to get a stastically significant and reliable standard deviation?

The same question could be asked in any number of situations in which a standard deviation is necessary for statistical analysis such as a Gage R&R study which requires a historical standard deviation.

Also, how do you go about estimating or conjecturing a standard deviation for such statistical analysis if you have little or no information as to what it might/should be?

Thank you in advance for any help.