Population Projections Martha McMurry State Demographic Center May 27, 2009

Past Errors in State Demographic Center Projections

Accuracy of projections is usually lower…… Over longer time periods than over shorter ones. In fast-changing areas; most accurate in areas of slow, steady growth. In smaller areas than in larger areas.

Projections error and Population Size (from Smith, Tayman and Swanson), over 10 years States: 4% to 8% Counties: 8% to 14% Census tracts: 15% to 21% For smaller areas (e.g. block groups) errors are even larger.

Sources of Error: Migration This is usually the largest source of error, especially for small area projections. Migration is affected by job creation, but many other factors are involved. Even if people move for job-related reasons, they don’t necessarily live close to workplace. Immigration/immigration policy another big unknown.

Sources of Error: Fertility Total fertility rates used have varied from about 1.85 to 2.1. Downward shifts are also possible. The effects of errors in projecting fertility become magnified with time. Mainly affects projections of younger population.

Sources of Error: Mortality Mortality rates in recent years have fallen rapidly. Will this continue? Pandemic or other disaster could result in rising mortality. To date, errors in projecting mortality mainly affect projections for older age groups.

Additional factors in small areas Zoning and zoning changes; development plans and actual development. Infrastructure can affect location of population. Small area estimates and projections are known to have high margins of error.