The option of error bars is available whenever replicate information is available for one or more data points.

This **Set
Error Bar Type and Level** option opens the Error Bars dialog.

This **Show
Error Bars** option toggles the error bars on and off.

Student-t Based Confidence Interval

The As Conf Interval selections offer an interval for the mean of the n entries comprising a given data point. Since the mean converges to the true value as n goes to infinity, confidence intervals are often thought of as the limits about the true value.

Student-t Based Prediction Interval

The As Pred Interval selections offer an interval for the next observation, based on the n entries comprising a given data point.

Simple Multiples of Standard Deviation

The As ± SD selections offer the simple SD multiples often used for error bars.

Normal Distribution Prediction Intervals

The As ± SD (%Pred n=Inf) selections also offer simple SD multiples. These are based upon the prediction intervals computed using a normal distribution (a Student-t distribution with n equal to infinity).

Just as the intervals about a fitted curve require normally distributed residuals, the error bar intervals require normally distributed errors within the replicates comprising each point in order to be fully valid. Since the n for each point representing a replicate is often small, and the confirmation of normality is seldom made, error bar intervals should generally be regarded as approximate. This is particularly true for 99% and higher levels where deviations from normality are more pronounced.