Figure 3 Complexity of order-picking systems (based on Goetschalckx and Ashayeri 1989)
Order picking objectives
The most common objective of order-picking systems is to maximise the service level subject to resource
constraints such as labour, machines, and capital (Goetschalckx and Ashayeri 1989). The service level is
composed of a variety of factors such as average and variation of order delivery time, order integrity, and
accuracy. A crucial link between order picking and service level is that the faster an order can be retrieved, the
sooner it is available for shipping to the customer. If an order misses its shipping due time, it may have to wait
until the next shipping period. Also, short order retrieval times imply high flexibility in handling late changes
in orders. Minimising the order retrieval time (or picking time) is, therefore, a need for any order-picking
system.
Figure 4 shows the order-picking time components in a typical picker-to-parts warehouse. Although various
case studies have shown that also activities other than travel may substantially contribute to order-picking
time (Dekker et al. 2004, De Koster et al. 1999a), travel is often the dominant component. According to
Bartholdi and Hackman (2005) ‘travel time is waste. It costs labour hours but does not add value’. It is,
therefore, a first candidate for improvement.