After nearly 20 years in the think tank business, one thing I’ve learned is that while philosophy matters, details matter more.

Changes in public policy do occur on the macro level. Broad shifts in sentiment, among policymakers and the public, can translate into specific legislative or regulatory actions. But many of the most significant public-policy outcomes occur from special-interest lobbying, legislative maneuver, redefining terms, and manipulating data — events at the micro level.

A good case in point is what’s going on in New York City with poverty statistics. The city is about to rewrite the definition of poverty, almost certainly to expand the number of residents considered to be “poor.” There is a longstanding dispute about flaws in the federal government’s original poverty measure, which was a bit of an improvisation back in the 1960s. Critics of the poverty-line calculation have just enough of a legitimate argument to open the door to great mischief. There ought, indeed, to be a poverty measure that takes into account a basket of goods and services, the often-significant differences in cost of living among states and metro areas, the value of government transfers, and other factors.

However, the best available evidence suggests that a properly defined, carefully structured poverty measure, using household expenditures rather than income estimates, would likely yield a lower poverty rate than the current one does (which would comport with the evident reality of improving living standards). But those seeking to redefine poverty have expansion, not accuracy, as their objective. Expect the new NYC measure to define many more people in “poverty,” for other jurisdictions to take note, and for a Democratic Congress and Hillary Clinton administration, should that occur, to make serious moves to implement a broader measure for the country as a whole.

It’s a matter of detail with potentially huge consequences for taxpayers, freedom, and economic progress.