Interval plot

A graphical summary of the distribution of a sample that shows its central tendency and variability. The default interval plot display consists of a mean symbol with a 95% confidence interval bar.

Interval plots are especially useful for comparing groups. For example, the following plot compares the breaking strength of steel rod samples produced by four different methods.

While the means appear to be different, the difference is probably not significant because all the interval bars easily overlap.

Here the difference between means probably is significant because the interval bars do not overlap.

Variability can be represented in an interval plot by two types of bars:

· Interval bar (default) - a vertical line with horizontal lines at the endpoints of the 95% confidence interval for the mean. Confidence levels can apply to the individual intervals or the family of intervals (Bonferroni).

· Standard error bar - a vertical line with horizontal lines at one standard error (or multiple of the standard error) from the mean. The standard error of the mean is calculated by dividing the standard deviation by the square root of n.