It is also worth emphasising that the models which have separate growth rates in each period (for example, as shown at the top of the section) are not wrong, but are just inconvenient from a sensitivity analysis point of view.
In many practical cases, one may in fact which to use figure for earlier periods which are different to those in later periods. For example, one may have specific knowledge about short-term budgets and plans that should be reflected in the model, whereas the longer-term part of the forecast may be more generic. Thus, it is possible to use “mixed” approaches i.e. where the short-term forecast uses separate assumptions for each period, whilst the monger-term part of the forecast uses a single assumption.