Comparing anything equally, fairly and transparently requires standards. Whether it's an asset class (equity, fixed income, etc.) or investment style (value, growth, etc.), the key to gauging performance and/or crafting portfolio diversification lies with standardization.

Impact investing is no different.

Sure the scrappy world of private equity, under which umbrella most impact investments fall, is difficult to capture in any uniform fashion. But it doesn't mean that standards shouldn't apply and sustainability reports shouldn't be mandated.

A solar power company in Kenya might have a different market, construct and corporate strategy than a microfinance company in rural India, but that doesn't mean a matrix shouldn't be available for comparative analysis.

Hard numbers that measure social change are what's need to create mass appeal.

Impact investing needs indexes, benchmarks and uniform reporting in order for investors to make intelligent investment decisions. Otherwise, it's investing in a vacuum--and that won't do the world as much good over the long term.

It's happening.

The Global impact Investing Network has developed Impact Reporting and Investment Standards (IRIS). This is a common framework for measuring social and environmental impact of investments.   

"One of the limitations to the growth of the impact investing industry is the lack of transparency and credibility in how funds define, track and report on the social and environmental performance of their capital. This leads to higher transaction costs and a limited ability to understand the impact of investments," the GIIN says.

To address these challenges, the GIIN is expanding upon work initiated by The Rockefeller Foundation, Acumen Fund  and B Lab  to develop and promote a common framework for reporting the performance of impact investments. The group, in collaboration with other impact investors and industry experts, has developed a standard set of performance measures for describing social and environmental performance that facilitates comparisons of impact data across investments.