Dear @HZhang
Thanks for your question.
I want to share my suggestions. Please follow them:
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As Prof. Sudret comprehensively discussed in the Acceptable LOO Error, the LOO error could have bigger values and its dependent on the case study. I think the trend of the LOO error is acceptable in your mentioned figure.
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What is the guideline? You could read my discussion on the Goodness-of-fit Tests question.
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Besides, based on the suggestions by the UserManual_PCE, please read my answers in the Polynomial chaos expansions question to use the ADAPTIVE options which are implemented in the UQLab.
I hope this helps but please let me know if you have any other questions.
Best regards
Ali