Page 345 - Design for Six Sigma for Service (Six SIGMA Operational Methods)

Design and Improvement of Service Processes—Process Management 305

patterns change constantly over time. For example, queues may form during
processes, such as those in manufacturing, when one process step or
operation is working faster than the next operation in the sequence. If there
is any variability in the operating rates, then the length of the queue may
also fluctuate.

Dynamic interactions make it so that we cannot analyze one process step in
isolation of the others, because many of the occurrences at one process step
may have been caused by another process step located upstream (or even
downstream). By the same token, any event at a process step may have
ripple effects, through dynamic interactions, both upstream and downstream.
Process starving is one type of dynamic interaction where one process step
cannot work even though it is in good operating condition, because an
upstream operation is not providing its needed input. Process blocking is
another type of dynamic interaction where an operation has to stop because
it cannot send on its output due to a problem downstream. Also, the stack-up
of variation on a part as it flows through a process where each operation
modifies a particular attribute on the part represents another type of dynamic
interaction. To obtain the variability of the overall process, we cannot
just sum up the variability of each operation as if the variabilities were
independent. The interaction effects must also be accounted for.

By considering the time dimension of a process, we can observe the manner
by which it evolves. A process is composed of one or more steps used to
transform some input into one output or a series of outputs. Each step or
operation may take some time to complete; this is the duration of the process
step. By observing the process in detail, a number of discrete points in time,
called event times, can be identified when certain changes occur in the
process. Events may, for example, include the starting and ending times for
each process step. Milestones are also event times and are highlighted in
order to focus on the occurrences at that event time. For example, the
midpoint of a project may be considered to be the milestone point at which
project progress is reviewed and certain adjustments can be made.

10.2.3 Process Resources and Constraints
A central issue in the design of a process is determining what tools and
other resources are needed for the process to be at its best. Resources are
generally defined to include key personnel with specified skills, equipment
with specified capabilities, and ancillaries necessary for the process to
operate. Such ancillaries may include space facilities or certain consumable
materials that are needed.