- https://scholar.google.com/ الباحث العلمي .
- https://www.ivsl.org/ivsl?func=feedback&language=ar المكتبة الافتراضية العلمية العراقية .
- https://www.base-search.net/ BASE .
- https://www.loc.gov/ Library of Congress .
- https://www.refseek.com/ Refseek .
- http://b-ok.org/ .
- https://www.pdfdrive.com/search?q=panel%20data&pagecount=&sortby=&searchin=&r=1 .
- https://core.ac.uk/ .
- http://academicindex.net/ .
- https://www.springeropen.com/journals .
- https://www.elsevier.com/about/open-science/open-access/open-access-journals .
- https://ieeexplore.ieee.org/Xplore/home.jsp .
- https://www.iasj.net/ .
- https://www.ncbi.nlm.nih.gov/ .
- https://www.jstor.org/ .
- https://www.tandfonline.com/ .