Which Academic Search Systems are Suitable for Systematic Reviews or Meta-Analyses? Evaluating Retrieval Qualities of Google Scholar, PubMed and 26 other Resources, October 2019.

“Our tests revealed that the help files of numerous search systems promise a Boolean search functionality that our tests could not verify. These findings were especially alarming because users of such systems rely on functionalities that they assume work properly, but that may not be the case.” … “our results contradict systematic review guidance that assumes that “all the search engines in some way [would] permit the use of Boolean syntax operators to expand or restrict the search””.

Regarding… “full Boolean search strategies” the authors also noted that “Google Scholar [does] not offer such functionality”. The word “full” here is the critical word, and indicates that NOT is still a missing operator for Google Scholar.

For open access, this new test concludes that those outside of biomedical research… “are limited to the multidisciplinary system BASE” for discovering open access material, but that unspecified… “other open, or partially open search systems that fail to meet the criteria for query-based search might still be useful for supplementary search methods.”