Every funder wants a number. Social Return on Investment gives them one, for example a 4.3:1 ratio, means every unit invested is said to return 4.30 in social value and that number travels. It goes into board papers, annual reports and grant applications and somewhere along the way it stops being a finding and starts being a fact.

The trouble is that the ratio is the least interesting thing a SROI produces. To get to it, you have to decide what counts as an outcome, whose outcome it is, how long it lasts and what would have happened anyway. Each of those is a judgement, defensible, but a judgement. The number is only as honest as the assumptions stacked underneath it.

This is not an argument against SROI. Done well, the process is one of the most useful conversations an organisation can have. It forces you to say out loud and in front of the people affected, what you actually think you are changing and how you would know.

That conversation is the real output. The figure is just where it happens to land; a single number standing in for a year of careful, contested, deep human reasoning.

So when someone shows you a ratio, ask the better question. "What did you have to believe to get there?" The answer tells you everything that the number on its own cannot.

Measure what matters. Then be honest about the measuring.