When Using Rank Regression (Least Squares)

When using grouped data, Weibull++ plots the data point corresponding to the highest rank position in each group. In other words, given 3 groups of 10 units, each failing at 100, 200, and 300 hours respectively, the three plotted points will be the end point of each group, or the 10th rank position out of 30, the 20th rank position out of 30 and the 30th rank position out of 30. This procedure is identical to standard procedures for using grouped data [19]. In cases where grouped data is used, it is assumed that the failures occurred at some time in the interval between the previous and current time to failure. In our example, this would be the same as saying that 10 units have failed in the interval between zero and 100 hours, another 10 units failed in the interval between 100 and 200 hours, and in the interval from 200 to 300 hours another 10 units failed. The rank regression analysis automatically takes this into account. If this assumption of interval failure is incorrect and 10 units failed exactly at 100 hours, 10 failed exactly at 200 hours and 10 failed exactly at 300 hours, it is recommended that you enter the data as non-grouped when using rank regression, or select the Use all data if grouped option from the Folio Control Panel's Set Analysis tab.