``Facts are stupid things,'' President Ronald Reagan once famously remarked. When it comes to making public policy, Boston too often follows the same philosophy.

In Boston, good information is a rare commodity. Data about the city are old, inaccurate or simply nonexistent. Lacking good numbers, policymakers rely instead upon anecdotes and intuition.

The results aren't pretty.

Consider housing. Boston Mayor Thomas Menino spent the week in Los Angeles trying to persuade his fellow Democrats that affordable housing is a crisis deserving national attention.

Menino makes that claim because affordable housing is such a crisis in Boston.

Or is it?

The city's Department of Neighborhood Development, which purports to track such things, just released a three-year analysis of rental housing costs. The report was accompanied by the oft-heard claims that rental costs were skyrocketing.

One small problem. The city's numbers don't seem to support that claim.

According to the city, monthly median rental prices climbed from $1,325 in 1998 to $1,465 in 2000. That's an average annual increase of 5 percent.

Five percent. This is a crisis?

For the last 30 years, the average inflation rate has been 5 percent. Admittedly, the 1990s saw inflation drop, to an average of 3 percent.But still, that means housing is climbing by a scant 2 points over the average - and a big chunk of the increase is the city's own fault, by the way, since it relentlessly boosts property taxes each year by 2.5 percent.

So does that mean there isn't a crisis?

Examine the city's numbers more closely, and they simply become more puzzling.

According to the report, it's more expensive to rent in the hardscrabble community of Roxbury ($1,500) than it is in relatively bucolic West Roxbury ($1,150). The report says that rents in central Boston dropped from $2,000 in 1998 to $1,400 in 2000, making what once was the most expensive part of the city now one of its cheapest.

The other item the city tracks is the number of apartments actually available. Turns out this is a crisis as well. In the first quarter of1998, 2,168 apartments were advertised for rent. Now? Only 1,311.

The sky is falling! The sky is falling! No one can find a place to live!

Please. Does anyone seriously believe that people willing to pay $1,500 for an apartment actually can't find one and are hence living on the streets?

Indeed, if available rentals really were in such short supply, one would expect prices to have climbed astronomically. As every car owner knows, that's what happened to gasoline prices when OPEC reduced its output. But the city's numbers don't show that.

So why the drop in available rentals?

The city's report on rentals is based solely on a reading of advertisements in one Sunday newspaper. Maybe realtors are using other means, such as the Internet, to advertise properties. Maybe people aren't moving as frequently. Or maybe it means nothing at all.

The point of this is not that there is no housing crisis. The point is that we don't know.

Right now, housing data are haphazardly collected by wide variety of city agencies, including the Department of Neighborhood Services, the Police Department, the Fire Department, the Assessing Department, Code Enforcement, the Boston Redevelopment Authority, and the Traffic Department, to name but a few.

Yet we know almost nothing. The city's obviously flawed report on rentals is the only effort the city makes to figure out housing costs.But it's worse than that. We really don't even know how many people live in the city. We think the population is growing, but we don't know who is moving in and who is moving out.

These are not trivial issues. When the data are lacking, people guess.

For instance: The city provides significant subsidies to help build low-cost housing. That policy has been driven by an assumption that the average low-income family has four to a household, meaning that it needs about 1,200 square feet in which to live.

Data just released this month, however, suggest that fully 75 percent of those with incomes under $50,000 are in one- or two-person households. Those families can easily get by with 700 square feet.

Oops. Looks like we've been subsidizing the wrong kind of housing.

The sense most everyone has is that, particularly in recent years, Boston has been undergoing some profound demographic changes. But in order to understand those changes, in order to respond to them, it needs information.

Right now that information is lousy. It's up to the city to correct that. Rather than following Reagan's advice, perhaps it would be better for the Boston to follow that of the Greek sages: ``Know thyself.''

Tom Keane writes weekly for the Boston Herald. He can be reached at tomkeane@tomkeane.com.