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What’s with those unusual population projections for Allenstown, anyway?



Monitor staff
Tuesday, September 27, 2016

When the New Hampshire Office of Energy and Planning released estimates last week of what will happen to state, county and town populations over the next 25 years, Allenstown residents might have noticed something odd.

The state projected that Allenstown’s population is going to fall 10 percent over the next decade, but after 2025 it will start growing again.

Head-scratching ensued. Does state government know something town residents don’t know? Why isn’t a similar boomerang showing up in other area towns? And what’s so significant about 2025, anyway?

Well, don’t worry, Allenstown. It’s not you – it’s mathematical modeling.

“It’s not that somebody tells us there’s going to be a big development somewhere. We didn’t get to that level of detail,” said Ken Gallager, a planner with the office.

Population forecasts are a basic planning tool for governments and businesses since, as is often said, demographics is destiny.

The big news from the OEP study released Sept. 18 involved aging. It predicted that by 2040 the number of retirees (people over 65) in New Hampshire would be almost double the number of youngsters, defined as people under 15. This update on the “silver tsunami” striking our shores got most of the attention.

Also in the data, however, were estimates for the year 2015 and then every 5 years afterward, for all 235 of our cities and towns.

That’s a lot of numbers. Where did they come from?

The past extrapolated, Gallager explained.

On the assumption that what has happened in recent years is a good guide to what will happen in future years – a basic principle of demographics – a consultant hired by the OEP took the percentage growth for each town during the decade of 2000 to 2010 and combined using the “shift-share method” with the percentage growth from the 2010 census through 2015, the most recent year in which the state estimated town-by-town populations.

So all the population forecasts in future years are really answers to arithmetic problems that use population changes since 2000.

And therein lies Allenstown’s oddity, which comes as a result of the 2006 and 2007 flooding of the Suncook River.

Those floods were the result of incredibly heavy rains, poorly placed dams and bridges, plus a lot of loose geology and perhaps some ill-thought-out sand removal, as some claim. The floods were disastrous, and wiped out so many homes that the town’s population fell.

Between the censuses of 2000 and 2010, Allenstown went from 4,800 people to 4,300, an 11 percent drop that was the biggest in the state over that period.

It has since grown again, but under the calculations made in the OEP projects, the drop before 2010 partly overwhelmed growth since 2015. The interaction between those two calculations explains Allenstown’s dip; the fact that 2025 is the year of rebound is a mathematical anomaly.

What does that say about these projections – which are “just a model,” to use a common phrase which implies that mere mathematics is a poor guide for reality.

Other factors can be taken into accounts when projecting growth to increase accuracy, such as business and housing starts, birth and death rates, and most significantly for New Hampshire the numbers of people moving in and out of the state, which has been the deciding factor in our growth patterns in recent years.

Those factors are often used in demographic studies, but it would be extremely expensive to gather such information all the way down to the level of towns; just doing it at the level of counties is time-consuming and difficult.

Using past growth patterns as a sort of proxy for them is often quite accurate, until it runs into an anomaly like the Suncook River floods.

(David Brooks can be reached at 369-3313 or dbrooks@cmonitor.com or on Twitter @GraniteGeek.)