The plot points of the probability plot represent the likelihood of a product failing before time, t
. For right-censored or uncensored data, Minitab calculates the plot points using the following methods:
- Median rank method (default)
- Modified Kaplan-Meier method
- Herd-Johnson method
- Kaplan-Meier method
If the data contain tied failure times (identical failure times), either all points (default), the average (median), or the maximum of the tied points is plotted. If the tie involves failures and suspensions, the failures are considered to occur before the suspensions.
Each of these methods generates nonparametric estimates of F(t), the cumulative distribution function for the random variable T, which is time to failure.
For a sample of n observations, let x(1), x(2),...,x(n) be the order statistics, or the data ordered from smallest to largest. Then i is the rank of the I th ordered observation x(I). The formula for each method is as follows: