You may have heard that 20 percent of retail storefronts in Manhattan are vacant. You heard wrong. This is just one of those Numbers People Say, like that we only use 10 percent of our brains. It’s false, and you should not make policy based on it.

The 20 percent figure has been used to perpetuate the idea that Manhattan faces a crisis of vacant storefronts despite a strong economy, driven by greedy and/or unrealistic landlords holding out for tenants willing to pay ever higher rents. In response, there is a renewed push for commercial rent control, to ensure incumbent stores are offered lease renewal at a fixed race. The hope, I suppose, is that our city’s neighborhoods can be preserved in amber and look forever as they did back when New York was “better.”

(I should note — when we talk about how New York “used to be better,” we mostly mean that we used to be younger. Unfortunately, this problem cannot be addressed through a rent-control policy or any other public policy. But I digress.)

Anyway: Comprehensive data about retail vacancy in Manhattan doesn’t exist. And nobody has actually promulgated a 20 percent estimate of the phenomenon people claim to be decrying: vacant, closed, unoccupied retail stores.

As far as I can tell, the 20 percent claim originated in two places. One is a September 7 story in the New York Times which said, “A survey conducted by Douglas Elliman found that about 20 percent of all retail space in Manhattan is currently vacant, she said, compared with roughly 7 percent in 2016.”

But the “she” in that story is Elliman broker Faith Hope Consolo, and this month Consolo told the Commercial Observer, a trade publication, that no such report exists. This might explain why Elliman’s PR office never responded last month when I contacted them to request the report. (Consolo says she was misquoted in the Times, though Times reporter Corey Kilgannon told the Observer he’s confident he quoted Consolo accurately.)

The other source for the 20 percent factoid is a 2017 equity research report from Morgan Stanley, which found the amount of available retail space in certain Manhattan submarkets had doubled since 2012, to 20 percent. But there are two issues with using this report to claim 20 percent retail vacancy in the borough.

One is that “available” is not the same as “vacant.” An available space might be occupied by an operating store that has declined to renew a lease that hasn’t yet expired, or by a pop-up store, or by store with…