Minimum Sample Size Required

[froggeja]

I am a total novice...I do hope you can help!

100,000 people usually shop in Store X.
But, only 50,000 people shopped in Store X in Period Y.
10,000 people (of the 50,000) purchased Product Y.
Purchase Rate = 20%.

How confident am I in the results? What is the formula for this?

And, what minimum sample size did I need in order to be 95% confident in the results? Again, what is the formula for this?

I've gotten myself into all sorts of difficulties - sometime a little knowledge can be worse than being a novice!

Thanks for your help in advance.

 

[XOR LX]

Hi,

Looks suspiciously like a homework question?

Regards

 

[froggeja]

I'm afraid not. I run a start-up business and we have just run a campaign for a brand and I don't have anyone with any statistical experience supporting me.