7.4.2 - Confidence Intervals
Standard Normal Distribution Method Section
The normal distribution can also be used to construct confidence intervals. You used this method when you first learned to construct confidence intervals using the standard error method. Recall the formula you used:
- 95% Confidence Interval
- \(sample\;statistic \pm 2 (standard\;error)\)
The 2 in this formula comes from the normal distribution. According to the 95% Rule, approximately 95% of a normal distribution falls within 2 standard deviations of the mean.
Using the normal distribution, we can conduct a confidence interval for any level using the following general formula:
- General Form of a Confidence Interval
- sample statistic \(\pm\) \(z^*\) (standard error)
- \(z^*\) is the multiplier
The \(z^*\) multiplier can be found by constructing a z distribution in Minitab Express.
z* Multiplier for a 90% Confidence Interval Section
What z* multiplier should be used to construct a 90% confidence interval?
For a 90% confidence interval, we would find the z scores that separate the middle 90% of the z distribution from the outer 10% of the z distribution:
For a 90% confidence interval, the \(z^*\) multiplier will be 1.64485.