Projecting at local level:Projecting at local level:policy considerationspolicy considerations

Keith WoodheadUnderstanding Local Population Projections

SW Observatory8th September 2010

It’s important to produce the It’s important to produce the best technical projections we best technical projections we reasonably canreasonably can BUT BUT

It’s all too easy to get carried away with producing a beautiful result whilst forgetting why we’re doing it in the first place

So, what about the user?So, what about the user?You mean

people use this stuff?

What does the user actually want?What does the user actually want?Users are often dealing with many sources of

risk – population related change may be just one of many elements and isn’t always the most volatile one

Ideally the user needs as much of the following in the projections as you can give:

Certainty – don’t provide him/her with false certainty but also don’t confuse with too many high/low variations if these are not essential (projection error probabilities will be dealt with later)

Stability – don’t revise projections too often; you might want the best/latest projections to be used but users have many more factors to integrate them with

Continuity – as far as possible revise successive projections in the same direction – don’t ‘hunt’ about the target

Technical purity doesn’t always help – it’s all a question of balance

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Dealing with risk and uncertaintyDealing with risk and uncertainty“Models are to be used but not to be

believed” Henri Theil

Prediction errors: forecasting is like driving when looking only in the rear view mirror

Use a method appropriate to the geographical scale, time horizon and degree of demographic detail required the problem.

Unnecessary detail just increases risk of error – cohort survival models are great at the “in situ” demographic processes but need to be integrated with other models if migration is critical.

At smaller scales (ie sub District level) apportionment/ ratio or econometric methods may be more appropriate

Prediction Prediction errorserrors

We often don’t know the present, let alone the future – all projections extrapolate the past

ONS Actual & projected pop’n UK 1951-2074

Source: Shaw 2007