Modelling Rates of Change
One of the main points of modelling is to try to predict what will happen, that is, we try to model how things change with time. Time itself can be modelled as continuous or discrete, and this is one of the modelling decisions which often have to be made. Sometimes there is a natural time interval at the end of which something happens, for example the breeding season of a living population. In cases like these a discrete model of time is probably appropriate, with a time step equal to the time between breeding seasons. The variable whose rate of change we are modelling can also be continuous or discrete. For a discrete variable whose value after n time steps is X n , the change or increment during the nth step is ΔX n = Xn+1 − X n .
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