• Estimates - refers to current population. are calculated in lieu of of an actual census count and are used to update population figures of the last census

• Projections - refers to future population levels. projections indicate what population changes might occur, given assumptions inherent in the projection method and data - typically presented in ranges - it, then

A. Extrapolation

Extrapolative Forecasting - a method of prediction which assumes that the patterns that existed in the past will continue on into the future, and that those patterns are regular and can be measured. In other words, the past is a good indicator of the future.

Applications: good for developing baseline data. Not as good for assessing the future impacts of a policy change, unless historic data for a similar policy change is available.

Attributes & Limits - simple, cheap and often as or more accurate that complex theoritical models.

Process - plot data and observe patterns.

Key: having a good base of data and understanding the pattern within it--easier said than done

Techniques: best fit, ratios, etc

Theoretical Forecasting - modeling, or using a construct of how some subsystem of the world functions to predict how things will happen in the future. There are empirical models, which reduce the problem to calculations, and nonempirical models, which do not quantify the problem under scrutiny.

Largely involves modeling. Models specify the linkages between relevant variables and are nothing more than informed speculation about the behavior of some system.

Applications: many rational decisions are made on the basis of models e.g., the law of gravity. (discounting, certain pop models, etc)

Attributes & Limits - more complex than extrapolation.

Process - based on longer-term relationships and research.

Techniques: econometric models, cohort survival, Reilly’s gravity model, discounting

Inductive Forecasting - a set of methods in which the future state is predicted (by persons who have some knowledge that makes them likely to do this accurately), and then data and assumptions necessary to achieve this outcome are deducted.

The most common forecasting methods. Rely primarily on subjective judgement. (seat-of-the-pants planning).

Attributes & Limits - uses retroductive logic eg a future state is described and the retroductive logic is used to find data and assumptions consistent with the forecasted end state.

Process - 1. choose knowledgable persons to interview. 2. anonymity; types - Delphi, cross impact analysis, feasibility assessment, etc.

1. relative accuracy
2. type of population data available
3. quality of data available
4. scale of the analysis
5. length of projection period
6. budget & time

Comparison of Population Projection and Estimation Models

Source: Local Population and Employment Projection Techniques. Greenburg, et al. Center for Urban Policy Research, 1979.

Source: Local Population and Employment Projection Techniques. Greenburg, et al. Center for Urban Policy Research, 1979.

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October 21, 2003