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Demographic data can serve several purposes. At the least, they provide a picture of the population as it is, or has recently been, in respect of the feature or property at issue. Further, in so far as comparable data are collected over a period of time, they will reveal changes that the population is undergoing. Finally, as trends are recognized from the past, they provide a means of forecasting changes to be expected in the future; and the fuller the understanding that can be gained about past changes and the forces that have brought them about, the more reliable the forecast of the future is likely to be.

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