Impact investment funds must stop relying on “fuzzy thinking” about how much good they do, Bono said as he announced a tie-up with U.S. private equity company TPG to measure the social and environmental change they achieve.

Impact investing – which promises social and environmental benefits as well as financial returns – is growing, but the difficulty of measuring how much good it achieves has caused some major investors to be cautious.

A new company, Y Analytics, aims to bridge the gap between researchers and investors “to help decision-makers evaluate impact,” said a statement from the company

“To persuade the biggest institutional investors to commit their funds to tackling some of the world’s most urgent challenges we need to be as confident about the impact returns as we are about the financial returns – fuzzy thinking just won’t cut it,” added Bono. Bono launched his own $2 billion impact fund, Rise, with TPG in 2016.

Data from the Global Impact Investing Network (GIIN), a non-profit organisation that promotes impact investing, shows the number of social investment funds has quadrupled over the past 20 years to 200. GIIN estimates the industry is worth $228 billion, yet there is no standard global definition of what qualifies as an impact investment.

The Organisation for Economic Cooperation and Development has also called for more rigorous standards to prevent “impact washing” – where firms seek to disguise unpopular practices or overstate the impact of their investments.

By Sarah Shearman @Shearmans, Editing by Claire Cozens.