Monday, October 09, 2006

Apples and oranges 

A colleague and I are scratching our heads at this line in a piece on housing costs in the St. Cloud area.

Median monthly ownership costs in St. Cloud � which include monthly bills such as mortgages, insurance, utilities and association fees � rose 13.01 percent when adjusted for inflation from 2000 to 2005, according to Census Bureau data.

Yet median household income fell 7.23 percent, when adjusted for inflation during that time.

I assume this story is from the latest release of the American Community Survey of housing. The data for St. Cloud, which came out the morning the Times article was released, is here. Nowhere in this report does it say anything about median household income. And as I discussed here last month, there are dangers in mixing data from various samples. So, I did deeper. Here's the chart that ran with the graph, and it cites not one but four sources for the data. OK, I'm already a tad suspicious, because the comparison is between 2000 and 2005, and ACS did not survey the St. Cloud MSA before 2004.

The reporter was away last week but returned this morning and answered some questions for me about the data. Indeed she has compared two different surveys, which are not longitudinal. That is, the two samples are not measuring the income or housing of the same people.

I've checked the inflation adjustments, and the article she wrote has some other data that she uses to back up the idea that housing is a bigger share of budgets in St. Cloud than five years ago. As to median income falling, what she doesn't pick up is that the margin of error (roughly, the 90% confidence interval around the estimate) is $5,102. This is large enough to say that there has not been a statistically significant difference in the city of St. Cloud. This Word document explains how to do the test.

That's not to say the data aren't telling. What they confirm for me, along with all the other data I look at for this area, is that the families moving from St. Cloud city to the suburban areas (mostly, Sartell and Sauk Rapids, though a number have gone out to St. Joseph) are of higher income levels than the new families moving into the area. This is what happens from drawing on two different surveys to make comparisons.