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From (a) country by country data and (b) model predictions made by integrating equations 7 and 9, with coloured circles representing the initial conditions. The arrows in the phase portrait indicate the direction and magnitude of the changes 
and 
as a function of the two variables 
and 
themselves. Specifically the arrows are the vector 
given by equations 7 and 9. These arrows are the same in both sub-figures.
