Sample size terms : confidence interval & confidence level
Before determining how many people you need to interview in order to get results that reflect the target population as precisely as needed, there are two terms that you need to know. These are: confidence interval and confidence level.
The confidence interval (also called margin of error) is the plus-or-minus figure usually reported in newspaper or television opinion poll results. For example, if you use a confidence interval of 3 and 47% percent of your sample picks an answer you can be "sure" that if you had asked the question of the entire relevant population between 44% (47-3) and 50% (47+3) would have picked that answer.