Acknowledgment

We are grateful to Inge Helland for his inputs on this paper throughout the period. His guidance on the envelope models and his review of the paper helped us greatly. Our gratitude also goes to thank Kristian Lillan, Ulf Indahl, Tormod Næs, Ingrid Måge and the team for providing the data for analysis. We are also thankful to the reviewers for their comments which helped us to improve this paper.