It depends on the confidence level you wish to achieve. Suppose you want to poll a sample of people to determine the percent of people who vote for a bill. If you want a 95 percent confidence level of being within two percent, then you would have to poll 600 people.
Descriptions of confidence levels, confidence intervals, and a Sample Size calculator can be found at http://www.surveysystem.com/sscalc.htm#t鈥?/a>How many data points are needed to draw a statistically valid conclusion?
you can make a valid conclusion from any number of data points, 30 and more is said to be "large"
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