In ClinicalLeader, an interesting review (even if dated 2015) of top CROs.
Three facts are being highlighted:
-the number of M/A, building global leaders
-the urgent need for small CROs to specialize
-the strategic alliances signed between CROs and data analysis service companies
A more recent study (2017 but in fact updating a 2015 research) can be found at Outsourcing-Pharma, study limited to the US market.