DavidEduardo said:it is a principle of polling that is invloved here. If you do the test over with different people, and get the same results, and do it again, and get the same results, you have used replication to prove that the sample is large enough to faithfully represent "everyone" (called the "universe") and no greater sample is needed. In fact, in most cases, even 100 people is more than enough, but we overrecruit just in case a perfect balence of subsets does not show up.
Sampling is used in every aspect of American business and even politics and government. Much of the US Census data is done by a sample (the long form) and not a census, in fact.
I use sampling, market research, and hard audience data in my business and have for years.
Your point was clearly illustrated to me personally years ago when we did annual polling to make sure we were hitting our audience targets. The fact of the matter was this....
For four years the polling showed that in fourteen different demo categoires nothing...and I mean nothing...ever varied by more than four percentage points from the year before. This was with an annual nth-name sample of less than half of a percent of the universe. In the fifth year of this excersize, we were able to create an opt-in audience database. Since we now had captured hard data on our audience....we didn't need to do the polling anymore.
Guess what? In all fourteen instances, the opt-in demo numbers all came in within THREE percentage points of the prior year's polling data. Without going into minutiae, the bottom line was whether we were working with a sample of less than half of one percent, or a sample that actually turned out to be 71%, the numbers were identical from a statistical margin of error standpoint.