Statistics for Maintenance Modelling: Regression Modelling

# Assessing the Appropriateness of a Proportional Odds Model¤

Can you think of two explanations for x having a diminishing effect on the hazard as time goes by? Click on the "Check Answer" button to find out.

Either:

1. the system adjusts to the factors imposed on it and the system reaches some stable state or,
2. the factors operate only in the earlier stages.

## Testing Whether to Use a Proportional Odds Model

In the same way as for the models already discussed, to help identify whether a data set should be modelled using the proportional odds model one can plot the data.

If the data are grouped, the jth group of nj observed failure times having x = j and nj being large, plots of
(1 - j(t)) / j(t) against t for different groups should resemble multiples of the corresponding baseline curve, and hence multiples of each other. Again, plotting the logarithm of each axis may yield tidier curves, while merely shifting the baseline curve vertically.

See the case study later in this session for an example of using this plot (screen 13).

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