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https://www.mckinsey.com/business-functions/strategy-and-corporate-finance/our-insights/a-better-way-to-anticipate-downturns?cid=other-eml-cls-mip-mck&hlkid=43c6066168d24ce48770cda86c18e473&hctky=&hdpid=5b2ed585-5c55-4c19-a844-02c1a823aa91
http://www.theglobalipcenter.com/wp-content/uploads/2018/02/GIPC_IP_Index_2018.pdf
http://www.cesifo-group.de/ifoHome/publications/working-papers/CESifoWP/CESifoWPdetails?wp_num=6456&CESifoWP.search=+