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I am working on an experiment and I need to know how many samples (n) I need to achieve a margin of error (e) below 2%.

Looking through a statistics textbook they provide a calculation for e using z-distributions, but not t-distributions.

Replacing variables I concluded that e = (t

_{a/2}S)

^{2}/[itex]\sqrt{}n[/itex] where t

_{a/2}is the upper bound, S is the sample standard deviation.

Is is correct? Also, if so, is the value of e given as a percentage?

Lastly, from some preliminary tests (2 tests), the closer the initial tests results are to each other the smaller the error value (obviously). But I am concerned that a sample size of two is simply too small to definitively conclude that I am safely within my desired margin or error.

I need to conduct tests under a variety of conditions and the final number of tests performed may run into the hundreds, if not thousands, so it is vital that I do not perform more tests for any particular set of conditions that absolutely necessary. Any advice in this regard would be greatly appreciated.

Thanks in advance.