Friday, April 21, 2006

Fun With Numbers: Sloppy Journalism

Look at the headline and sub-head of this Tennessean story:
Tennessee growth slows in 2000-04:
State still has nation's ninth-fastest rate of growth, but fewer people are moving in than in the '90s
So, it's going to tell us our population rate of growth is slowing?

Not quite. Read the story and what you learn is that Tennessee's rate of growth by net in-migration is slowing, not the rate of population growth or the overall rate of growth. In other words, looking solely at the number of folks moving to Tennessee from other states and other nations versus the number leaving, we are slowing down.

One of these things is, clearly, not like the other, as the Sesame Street song goes. It may seem a small, picky thing (Go on; I know someone wants to say that.) but it's important.

If Tennessee is growing, but our rate of in-migration is slowing, then obviously more Tennesseans are having children. If Tennessee is losing people, and our rate of in-migration is slowing too, then we're in trouble.

But the story only looks at "net" change, the sum of leaving and entering. The more important numbers would be how many are choosing to leave Tennessee and how many are choosing to come here. For example, if the number of folks coming in to the state was constant during the period studied, but the numbers leaving was growing, then we'd have one kind of problem: More and more Tennesseans are finding Tennessee unlikable.

If it was a case where the numbers leaving were constant, meaning as we otherwise grew fewer were taking off for other climes, then the numbers of people moving in was declining, meaning fewer non-Tennesseans found our state attractive enough to move to.

As one data point in a meta-analysis of trends nationwide (which the Census Bureau report is), it tells us something. But as a picture of Tennessee, it doesn't tell us much.

What we do learn from reading the report itself is that for fifteen years Tennessee has been a net attractor. Look on Page 7 of the report and you'll see the South as a whole is doing pretty well.

Dig deeper into the report and you learn something truly interesting if you're a Mid-Southerner. On Page 11 is a map of the country by county looking at migration trends over the last fifteen years. Looking at Tennessee, you see that a broad area around Nashville has been growing (in-migration wise), as has the area around Knoxville.

The Mid-South is a very different story. In fact, Shelby County is an oasis of slower out-migration along the Mississippi River counties' big losses! What's traditionally thought of as "the Delta" or "the lower Mississippi" is hemorraging migration growth.

Not only that, but the whole Gulf of Mexico is growing in migration except the coast along Louisiana. And, if you look at the map on Page 12, you'll see that Nashville has lost significantly in the past four years while the area around it is growing modestly. (Again, remember, in migration-only growth.) The West Tennessee area is a more mixed bag.

Now, Tennessee is growing, by 220,000 in the last four years. Don't worry, even if pure population growth (by all reasons) has gone way done. (By three-quarters!)

We are still Number 16! Whoo! And if you want a more hopeful view of the longer term, try here.

Well, the big point I wanted to make is to be careful slinging numbers around. Read carefully when some journalist starts talking about numbers, especially from a press release from some other agency or group! Most journalists have little or no math skills, and few have ever studied statistics. Many lack critical analysis skills.

Reader beware.

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