Lüke, T., Wilbert, J., Weichselbaum, M., & Grünke, M. (2014). On the visibility of “Empirische Sonderpädagogik”: A bibliometric analysis. Empirische Sonderpädagogik, 4: 365-372
The most interesting thing about this study is the data about the visibility of seven German Education and Psychology journals that publish articles on Special Education (2010-2013).
These journals are all indexed on Google Scholar. Only one of them is present in Web of Science, and two of them are in Scopus. Another example of the wider coverage and size of Google Scholar, compared to traditional databases.
The average indexing rate of the articles published in these journals on Google Scholar is 74%, but there are significant differences between them. Three of them are covered entirely (100%) while for the other four, less than 50% of the articles are covered. One of them is practically invisible (11%). Therefore, there is still a number of documents that Google Scholar hasn't been able to discover.