Determining sample size
===

If I have a population and want to draw samples from there, what are the steps to determine the number of samples to draw from the population? In other words, if I have population N and want to draw k samples from them so that the samples are representative of the entire population, how can I determine k?

I am having some insights from [here][1] but in the end of this document I find- 

> You can still use this formula if you don’t know your population standard deviation

which is my case (i.e., I do not know the population variance). How can I use that formula if I don't know the population standard deviation/ variance? 

n.b. I posted this question to that forum but still no luck.


  [1]: http://www.isixsigma.com/tools-templates/sampling-data/how-determine-sample-size-determining-sample-size/