Donors give for many reasons.  Most donors, though, do seek some sort of return on their charitable investment. Of course, the level of that social return can vary widely. Determining impact, however, can be extremely challenging. Donors have limited time and likely contribute to a number of causes.

Arthur Brooks will be stepping down as American Enterprise Institute’s president after a decade at the helm, it’s a good time to turn to his thoughts on the matter. His 2006 book, Who Really Cares, caused a massive disruption in the public perception of the demographics of U.S. donors.

As a donor, perhaps you’ve witnessed two of the great measurement errors nonprofits make. The first, Brooks notes, comes when a group argues that “our work is unique and our organization is so unlike anything else that exists that we can’t compare it with—or measure it against—any other organization.” The second error is what he dubs the “lamppost error,” the notion that the nonprofit simply measures whatever is easiest.

Truly measuring impact in the public policy and ideas space requires a different strategy. It’s important to measure the right component of an output.

It’s also worth noting that driving change from the ideas world takes time. AEI could identify public policies it has “created,” or suggest that US macroeconomic conditions rise and fall according to AEI’s research. However, these alone would be disingenuous claims given the array of factors that affect political change.

Donors, then, can’t expect a public policy organization to change the world overnight. They can and should expect an organization to move consistently forward and offer clear evidence that it is trying to do so.

Read more about measuring impact by Peter Lipsett at Philanthropy Daily.