Click the graph to select it, then click the plus sign beside the graph and select items to display on the graph. The following information describes some of the items on the Graph Elements menu.
Values on the y-axis represent estimated cumulative percentages. The estimated cumulative percentage is equal to the estimated cumulative probability multiplied by 100.
Values on the y-axis represent estimated cumulative probabilities. The cumulative probability for a value x is the probability that a random observation that is taken from the population will be less than or equal to x.
Minitab uses the median rank method (also called the Benard method) to estimate the cumulative probability (r) for each observation:
In this formula, i is the rank of the observation in the sample and n is the total number of observations in the sample. For the smallest value in the sample, i = 1 and for the largest value in the sample, i = n.
Values on the y-axis represent inverse cumulative probabilities.
The score values for the normal distribution and the lognormal distribution are the inverse cumulative probability of r, calculated using the standard normal distribution.
The score values for the exponential distribution and the Weibull distribution are calculated as LN(−LN(1−r)), where LN is the natural log function.
Add a fitted distribution line to assess whether your data follow a specific theoretical distribution. For example, many statistical analyses assume that data follow a normal distribution. Data that fit the distribution well have bars that closely follow the fit line. Minitab also estimates the distribution parameters from your sample and conducts an Anderson-Darling test to test whether your data fit the theoretical distribution.
To fit a lognormal distribution, an exponential distribution, or a Weibull distribution, all data values must be greater than 0.