Can long-term forecasters answer these questions?

I find it difficult to believe some in the media are taking these latest economic forecasts for 15 years outside the EU seriously. They have all the hallmarks of the approach that the Treasury used to get the short-term forecast for the aftermath of a Brexit vote so hopelessly wrong.

The first thing to stress is the forecasts which state the UK as a whole will lose 2 percent of GDP if we stay in the single market, 5 percent if we leave with a trade deal, and 8 percent if we leave without a trade deal are not saying we will be between 2 to 8 percent worse off in 15 years time. This is an estimate of slower growth, not an absolute decline. If we carry on growing on average at 2 percent per annum over the 15 years, we will be 34.6 percent better off at the end of the period. These forecasts suggest that might only be 32.6 percent