We have been using New Relic, I want to have an idea, as what is the sample size in apdex.? And how does it relates to the actual visitors served by the web site? Supposing, there isn't any cache mechanism installed, then sample size will be actual visitors served?
New Relic apdex isn't calculated based on a sample -- it is based on all the visits within a minute time period.
But one thing to remember about Apdex is that it isn't related to throughput. Your throughput could drop in half and your Apdex could remain high.
The way I think of Apdex is that it is a measurement of customer happiness, and that it is sensitive to deviations from the average. It is a bit like standard deviation in that it shows you how your site performance is affecting real people.