Question 275995: What is confidence interval?
Answer by mathjames(8)   (Show Source):
You can put this solution on YOUR website! Confidence intervals are constructed at a confidence level, such as 95%, selected by the user. What does this mean? It means that if the same population is sampled on numerous occasions and interval estimates are made on each occasion, the resulting intervals would bracket the true population parameter in approximately 95% of the cases. A confidence stated at a 1 - alpha level can be thought of as the inverse of a significance level, alpha. In the same way that statistical tests can be one or two-sided, confidence intervals can be one or two-sided. A two-sided confidence interval brackets the population parameter from above and below. A one-sided confidence interval brackets the population parameter either from above or below and furnishes an upper or lower bound to its magnitude. For example, a 100(1-alpha)% confidence interval for the mean of a normal population is;
Ybar +/- (Z(alpha/2)*sigma)/SQRT(N)
where Ybar is the sample mean, Z(alpha/2) is the upper alpha/2 critical value of the standard normal distribution which is found in the table of the standard normal distribution, sigma is the known population standard deviation, and N is the sample size.
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