Is Population Growth Slowing?

I had to reach back a bit for this one, but this is one of the things that gave me the idea of writing this blog.  It’s a blog from New Jersey Future (who I like), and it looks at how population growth is changing from one Census to another, with a focus on New Jersey.

The article, Population Growth Slows in NJ, Nationally, claims that “the country’s rate of growth in the 2000s — 9.7 percent — is its slowest rate of growth over a decade since the Great Depression (the US population grew by only 7.3 percent in the 1930s)”.  To be fair, NJ Future is not the originator of this idea, but I read them regularly, so it’s where I picked it up.

This is a classic case of comparing rates of change where the base varies too substantially to be meaningful. Looking at the chart below, the U.S. actually added the third most people in a decade since 1910 and 2000 to 2010 is essentially equivalent to 1950 to 1960 (second most). Only 1990 to 2000 stands out above all other decades.

Turning to New Jersey, the first decade of this century showed relatively less growth than in the past. The following chart shows how the NJ population changed – in absolute numbers – for the Garden State. New Jersey added about half as many people from 2000 to 2010 than from 1990 to 2000. The 2000 to 2010 growth was about 100,000 fewer people than the average of the past 10 decades. Comparatively, the U.S. as a whole added almost twice as many people from 2000 to 2010 than the average of the past 10 decades.

Once again, its easy to look at a percentage and come to false conclusions. From an urban planning perspective – and New Jersey Future is an urban planning blog – decisions are impacted by numbers of people, not growth rates.

The original post also includes a Census map showing the percent change in population by state. I’ve reproduced that map, along with a companion map (on the right) showing actual change in population. A simple comparison illustrates how misleading percentages can be. Nevada grew 35 percent between 2000 and 2010, while California grew by ‘only’ 10 percent. This suggests that Nevada grew 3.5 times faster than California, right? Not so fast. California added 3.4 million people, while Nevada added 700,000. California added almost 5 times as many people in last ten years than Nevada. And while California is clearly a larger state, those people still require infrastructure and services, as well as contributing to the economy of the state.

You can click through to the Many Eyes Visualization below and look at any decade – you have to change both maps though.

State Population Growth 2000-2010 (pct on left, num on right) Many Eyes

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One Response to Is Population Growth Slowing?

  1. Tim Evans says:

    Your argument about my post on New Jersey Future’s blog is basically a wholesale argument against percent changes. If I were trying to mount a full-bore defense of percent changes, I’m not even sure where I’d start. To keep it simple, let me just say that people experience changes subjectively. Specifically, in many situations they perceive change relative to what was already there. The way you measure things as a comparison to what’s already there is by using percent changes.

    Let me give you an example. Pike County, PA, was one of the fastest-growing counties in the entire northeastern US in the 2000s, growing by 30.7 percent from 2000 to 2009. In absolute numbers, this translates to a little more than 14,000 people. Philadelphia County (which is coextensive with the city of Philadelphia) actually gained more than twice as many new residents (~29,000) over the same period. But which change do you think had the greater impact on the existing residents and their local governments’ capacity to provide services to the new residents: an addition of 29,000 people to a city that already had 1.5 million, or a new 14,000 people in a county that started the decade with only 46,000? To make your unqualified claim that “From an urban planning perspective…decisions are impacted by numbers of people, not growth rates,” you need to ignore the concepts of economies of scale and fixed costs, not to mention the more intangible idea of a learning curve.

    If you think percent changes are misleading — if you think citing a percent change automatically makes you fair game for inclusion on a blog devoted to “Bad Stats” — then you may want to take that up with the Census Bureau (my former employer, and the nation’s pre-eminent collector and distributor of data). They publish all kinds of changes in percent form. I hope you will reconsider your position on percent changes.

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