Opinion – Evidence, predictions and forecasts: what is the difference?

The difference between evidence, predictions and forecasts is important to many types of activity but seems to cause particular angst in the global warming (climate change) arena.
The easy part of this is “evidence”.  Evidence is about observing what has already happened whether the observations are directly of the natural world or of an experiment which attempts to mirror a part of natural world behaviour, and so on.  I will come back to evidence but the important point is that it is historical – it deals with things that have already happened.

Predictions and forecasts on the other hand are about the future – they’re concerned with estimating what might or will happen at some future time.  There is a huge and important difference between predictions and forecasts on the one hand, and evidence on the other.  One of the problems with the discourse on global warming is that this distinction too often gets lost – or more accurately mislaid!!
The difference between a prediction and a forecast is trickier.  If you consult a dictionary there does not seem to be much of a difference – they almost seem like synonyms.  But in normal science usage there is a distinct and important difference and I would characterise it as follows:

• Predictions are about estimating the future behaviour of a system based on certain assumptions or inputs and usually linking the two through some kind of model.  Some models are very simple because they describe very simple systems or parts of a system and in those cases the predictions are typically accurate and reliable. For example if you have water in a glass and heat the water up, you can predict with a high degree of certainty how much the water level in the glass will rise as a function of water temperature.  But if the system is complex and dynamic, modelling becomes much more difficult and at the extreme the models may be of little use or even misleading.  The climate system on planet Earth is an example of a very complex, chaotic and dynamic system which is still not well enough understood to be confidently modelled as a whole.

Before models are used they should be tested against history and that will generally give a good indication of how reliable they will be in looking forward.

• Forecasting is simply about estimating what will happen at a future time, and may or may not involve the use of predictive models.  Weather forecasting is an example of something which is a bit of a mixture of both.  The point about forecasting is that it tends to be unconditional, ie it generates statements like “I estimate this is what will happen in 10 years’ time”.   I don’t envy the weather forecasters because for all of the expertise and technology at their disposal they rarely get it exactly right, because they are dealing with a chaotic system.

For all of that people like forecasts because they are definite and because people actually like speculating about the future. And that brings us to the major issue with forecasting.  By and large we humans are very bad at forecasting the future and there are numerous examples of massive mistakes being made,  eg the Club of Rome predictions, the forecasting of peak oil in the 1990s and so on – and minor mistakes such as Treasury’s ongoing difficulties in forecasting the future performance of the NZ economy.

It would be nice to avoid forecasting but unfortunately we can’t as forward planning in many areas involves having some sense of future context even if predictive modelling is not particularly helpful or relevant as an input.  Just be aware of the provisos and pitfalls.

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This is another of Bas Walker’s posts on GrownUps.  Please look out for his articles, containing his Beachside Ponderings.