Population stability Index vs Population Accuracy Index
Population stability refers to whether the  the distribution
of explanatory variables is changing over time. When this distribution changes  there is more concern over whether the model is currently fit-for-purpose since the data used to develop the model differs from the data the model is being applied to.
The Prediction Accuracy Index (PAI) is defined as the average variance of the estimated mean response at review divided by the average variance of the estimated mean response at development. In PSI we don't take into account the predictive ability.
To summarise,the Population Stability Index (PSI), interprets whether the distribution of development data and the distribution of review data are similar.  The PAI checks the predictive ability of the model on current data is not significantly worse than the predictive ability at development.
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