Graph Error Bars (2D)
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.