## 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 . It is usually associated with the

*significance level*

=

The construction of CI is
based upon the sample mean
=
and the sample standard deviation
=
from data of sample size
*n*
=
.
The most commonly used confidence interval is a *two-sided CI*
which is centered at the mean
and extends either side an equal amount.

= ( , )

If the standard deviation

=

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

= ( , )

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