The past few years have been a busy time for mergers and acquisitions in the education industry, with companies seeking to strengthen, or in some cases diversify, their presence in the K-12 market, particularly in the digital world.

Which education companies have been most active players in acquiring privately held education technology companies over the past few years? CB Insights, a New York City-based business that analyzes and tracks the market, has taken a look.

It finds that Pearson tops the list, having made 15 acquisitions or investments in ed-tech companies from 2010-2013, followed by Blackboard and the Macmillan Group, each with seven. Other well-known names, like Kaplan and McGraw-Hill, also make the list, followed by others like Chegg and Rosetta Stone. (See the full list below.) Chegg is a Silicon Valley company that originally launched as a textbook rental company. It now offers an array of educational materials, from textbooks to homework help. Rosetta Stone is a provider of software to learn languages.

CB Insights’ list includes both public and private companies. The analysis points out that the list is focused on companies, and does not include private-equity firms and other players that could be making mergers and acquisitions.

Overall, there were fewer high-profile education mergers and acquisitions in 2012 than the previous year, as Education Week reported recently. About $7 billion changed hands in more than 250 total transactions in 2012, down from $12 billion in 2011, according to a report by the investment banking company Berkery Noyes.