*[https://www.brighthubpm.com/project-planning/100279-pestle-analysis-history-and-application/ A Review of PESTLE Analysis History and Application]
*[https://pdfs.semanticscholar.org/d9d2/86c5a903a91d4e5e6cff565f186f91383a02.pdf Environment & PEST Analysis: An Approach to External Business Environment]