Population Projections Martha Mc Murry State Demographic Center
- Slides: 10
Population Projections Martha Mc. Murry 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.
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