Regarding a Delta Think blog post analysing the DOAJ – News Service

"The main problem, when you compare ROAD, Web of Science (WoS), Scopus and DOAJ, is that all of these services have different definitions and criteria as to what constitutes a valid journal entry in their databases.

In general, one can say that DOAJ’s criteria are the strictest and therefore DOAJ is not an index of all open access (OA) journals but an index of gold standard, quality, peer-reviewed OA journals. So therefore not all OA journals meet our criteria.

Being indexed in DOAJ acts like a badge of quality. A quality stamp based on the business operations of a journal and its reliability, how closely that journal adheres to best practices and which standards it uses. Scopus and WoS are not in the business of measuring any of those. (We would take this opportunity to point out that DOAJ holds many more journals which aren’t in Scopus.)..."