## Confidence Interval

A confidence interval (CI) provides a range of plausible values for the unknown population mean . The choice of the confidence level is typically 90%, 95% or 99%, and represents the chance that the CI does indeed contain the true population mean . The construction of CI is based upon the sample mean and the sample standard deviation from data of sample size . The most commonly used confidence interval is a two-sided CI which is centered at the mean and extends either side an equal amount.

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If the standard deviation is known, the critical point should be replaced by of the standard normal distribution.

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