## Basic Confidence Intervals

Assuming that the population is normally distributed,
the **Confidence Intervals** table
gives confidence intervals for the mean, standard deviation,
and variance at the confidence coefficient specified.
You specify the confidence intervals either in the distribution output
options dialog or from the **Tables** menu.

**Figure 38.10:** Basic Confidence Intervals Menu

The confidence interval for the mean
has upper and lower limits

where is the critical value of the
Student's *t* statistic with *n*-1 degrees of freedom.
For weighted analyses, the limits are

For large values of *n*, acts as ,the critical value of the standard normal distribution.

The confidence interval for the standard deviation
has upper and lower limits

where and are the and critical values
of the chi-square distribution with *n*-1 degrees of freedom.
For weighted analyses, the limits are

The confidence interval for the variance
has upper and lower limits equal to the squares of
the corresponding upper and lower limits for the standard deviation.

Figure 38.11 shows a table of the 95%
confidence intervals for the mean, standard deviation, and variance.

**Figure 38.11:** Basic Confidence Intervals and Tests for Location Tables

Note | The confidence intervals are set to missing if vardef**DF**. |

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