LogLikelihood Ratio test for POD ceiling

It is unwise to assume that the POD vs size plot always goes to POD = 1 without verifying that the data support that assumption. Datasets included with the STANDARD mh1823 POD package include the infamous NTIAC 1997 A9002-3-L.  The standard analysis produces a warning:

WARNING - POD(a) model is wrong!

The STANDARD software uses a preliminary test for floor or ceiling: Does the 90% POD catch 90% of the cracks larger than a90?  This is a very basic test and can be unreliable.

The profile loglikelihood plot shows the maximum achievable loglikelihood for a series of possible values for the upper POD asymptote (“ceiling”).


Profile loglikelihood plot showing upper POD asymptote = 0.84

Profile loglikeliihood plot for infamous ceiling

With the workshop Edition software making a profile loglikelihood plot is straightforward:  Choose Use 3-parameter POD(a) function from the menu.  This will open another menu:

Make to plot and complete your analysis using methods discussed on the Workshop.

Floor - Ceiling menu

Completing the analysis:

Correct POD(a) plot

The Workshop Edition software also finds POD “floor.”