Operational improvements, rather than debt, have fuelled returns at private equity-backed businesses sold since the financial crisis, according to a new study which challenges the view that leverage has driven returns from boom-era deals.

Returns from companies bought by buyout firms during the boom of 2005 to 2008, that have since been sold, have outperformed the public markets, according to a new study by Switzerland-based private equity investor Capital Dynamics and Technische Universität München.