  The hierarchical structure of production planning has the advantage of
assigning different decision variables to their respective time horizons and
therefore ensures their manageability. However, the restrictive structure of
this top-down approach implying that upper level decisions are the constraints
for lower level decisions also has its shortcomings. One problem that occurs is
that deterministic mixed integer decision problems are often used for long-term
planning, but the real production system faces a set of stochastic influences.
Therefore, a planned utilisation factor has to be included into this
deterministic aggregate planning problem. In practice, this decision is often
based on past data and not consciously taken. In this paper, the effect of
long-term forecast error on the optimal planned utilisation factor is evaluated
for a production system facing stochastic demand and the benefit of exploiting
this decision's potential is discussed. Overall costs including capacity,
backorder and inventory costs, are determined with simulation for different
multi-stage and multi-item production system structures. The results show that
the planned utilisation factor used in the aggregate planning problem has a
high influence on optimal costs. Additionally, the negative effect of forecast
errors is evaluated and discussed in detail for different production system
environments.
