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So forecasting difficulty levels is about as difficult as forecasting bitcoin prices!
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Not really. Forecasting bitcoin prices is impossible to do with measurable accuracy. Forecasting the network hashrate and difficulty is possible to do with some accuracy. I do weekly forecasts here:
http://organofcorti.blogspot.com.au/search?q=%22weekly+forecast%22The four week forecast network hashrate has been quite accurate recently:

You say quite acurate but in reality (the one you share with other people) the predictions are all overestimating by about 20%.
I mean, i can predict without any mathy that hashrate will increase in the near future. But the question is, by how much and when in time. If your models cannot predict that then they are useless for the purpose.
They don't add any information that could not have been obtained by simply knowing that ASICs wll be delivered.