Typically, they are assumed not to occur for the purposes of making formal forecasts. Even when changes are observed, their effects can in some cases be particularly difficult to quantify. Hence, they are normally listed under ‘risks’. This doesn’t mean we should disregard forecasts or try to make policy without them.
But it does mean that policymakers cannot assume that the forecasts will give a signal which is sufficiently reliable on its own to be the sole basis for policy. It also means that risks often feature more prominently in policy deliberations than do the forecasts themselves.