Are Census Projections Good Assumptions?

"Last September, the Census Bureau released its American Community Survey estimates for cities, counties, and census-designated places with populations over 65,000. Most notable was the ACS's published margin of error, which allows the data user to construct the low-to-high error range of the estimate.

Technically, any number within the range is correct, but the popular press had a field day with that definition of "correct." Reporters found cities where the low end of the error range was below the city's current population and the high end of the range showed a dramatic increase.

...Keep in mind that a projection is a conditional statement about the future based on a set of assumptions. A projection is basically math run on a set of data and assumptions. In applied demography, the beginning year census data and math are seldom wrong. The real art is in choosing assumptions, which are often implied future planning decisions.

...Finally, think ahead. If communities create entitlements to accommodate projection-based growth for 10 or 20 years into the future, that growth is likely to occur. In other words, growth will become a self-fulfilling prophecy.

Isn't it irresponsible to ignore projections in planning? Yes, it is. What is the alternative? The alternative is old-fashioned planning that takes projections into account, but is not driven by projections. Suppose planners asked these questions: Can we have our vision in the context of these population projections? What will happen if we try? How much and what kind of projected demand should we accommodate to keep our economy healthy and to house our own children?

That is a very different paradigm from this one: "We must plan to accommodate [number] housing units over [years] based on the [source] projection."

Chris Williamson, AICP, is a senior planner in Oxnard, California, and adjunct associate professor of planning and GIS at the University of Southern California. He also teaches Planetizen's PLAN-120: Introduction to Census Data, ACS, and Growth course.