Now,I'm not really concerned about the CTR, but the actual conversion ratio.
Most of are successful campaigns are seeing around 9-10% conversion. I want to be able to test for 1-2 weeks, and know with 95% certainty that the conversion rate I see will be within say, 1% (maybe .5%) of the results. How many samples (clicks) must I record to find this?
If you run what is known as a monte carlo analysis (statistical simulation) 200 samples is usually enough. However, in the case of marketing there can be delays from click to conversion. I have found that after 1000 clicks statistics start to converge to a realistic number. In some cases I have noticed a good conversion rate for new ads and then after running the ad for a while it slows down.
I have about 450 keywords in my Overture account, some bids are around 50p per click. You mean I'd have to spend £500 on 1 keyword before seeing whether it's converting or a waste of money? Wouldnt 100 clicks or less be enough?
Seasonality play a role so when you run a campaign is also something to consider (unless it's not a seasonal product). I think it's probably safe to assume that you'd get far fewer conversions two months away from Christmas as opposed to two weeks away.