It would not be a proper private markets sustainability conference if data – the gathering of it, reporting of it, verification of it – did not weave its way through every conversation.

Such was the chatter at last week’s Impact Investor Global Summit in London, where the push for standardised ESG and impact data was causing optimism and grumbles in equal measure.

Cecilia Chao, managing director of Bain Capital’s impact platform, described the process of gathering and reporting impact and ESG data from the firm’s mid-market portfolio as an “impact tax”: “I just want to make sure we don’t lose the plot… which is trying to drive impact outcomes.”

“We have got this absurd position whereby the attempts to create standardised ESG disclosures have created a whole bunch of disclosure frameworks that firms are being asked to disclose against. That’s pretty burdensome,” said Michael Johnson, a regulatory expert with KPMG.

And while limited partners of course want “non-financial” data, they can operate without it. Said AP4’s senior portfolio manager Hanna Ideström: “We will continue to put money [into impact funds] even in the absence of perfect KPIs and perfect verification of it.”

For all the difficulties, however, reliable comparable data is essential if capital is to be allocated sustainably and greenwashing is to be mitigated. “The solution is not to not collect data or not transparently report; the solution is to get to some sort of global harmonisation,” said EQT partner Rikke Kjær Nielsen.

Few people would disagree with this lofty goal. Data providers, investor working groups and non-profits are all working towards it.

The next major milestone on this journey will come courtesy of the US Securities and Exchange Commission. On Wednesday, commissioners approved a new proposed rule that would require investment advisers in the US to make disclosures about their use of ESG factors.

Happily, the proposal looks to be analogous with the EU SFDR: the amount of disclosure would depend on the categorisation of a fund as either an “integration fund” (where ESG plays a part in the investment process), an “ESG-focused fund” or a full-on impact fund. If the EU SFDR experience is anything to go by, defining these categories will not be simple.

The promised land of harmonised ESG data is a long way off; it may even be a mirage. Consider the GP-LP reporting of financial returns; this should theoretically be a pretty black-and-white, unambiguous exercise, but the most prevalent data point – the internal rate of return – has for decades been the subject of criticism for how variable its calculation can be.

Investors have grown used to comparing apples to ever-so-slightly-different apples when it comes to financial returns. Sustainability-focused LPs will have to take a similar approach.