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Chapter 7 Layout and flow 197
Total work content
The number of stages
The next decision concerns the number of stages in the layout and depends on the cycle time
required and the total quantity of work involved in producing the product or service. This
latter piece of information is called the total work content. The larger the total work content
and the smaller the required cycle time, the more stages will be necessary.
Worked example
Suppose the bank in the previous example calculated that the average total work content
of processing a mortgage application is 60 minutes. The number of stages needed to
produce a processed application every 15 minutes can be calculated as follows:
total work content
Number of stages =
required cycle time
60 minutes
=
15 minutes
= 4 stages
If this figure had not emerged as a whole number it would have been necessary to round
it up to the next largest whole number. It is difficult (although not always impossible) to
hire fractions of people to staff the stages.
Task-time variation
Imagine a line of four stages, each contributing a quarter of the total work content of processing
the mortgage, and passing the documentation on to the next stage every 15 minutes. In
practice, of course, the flow would not be so regular. Each station’s allocation of work might
on average take 15 minutes, but almost certainly the time will vary each time a mortgage
application is processed. This is a general characteristic of all repetitive processing (and
indeed of all work performed by humans) and can be caused by such factors as differences
between each product or service being processed along the line (in the mortgage-processing
example, the time some tasks require will vary depending on the personal circumstances of
the person applying for the loan), or slight variations in coordination and effort on the part
of staff performing the task. This variation can introduce irregularity into the flow along the
line, which in turn can lead to both periodic queues at the stages and lost processing time.
It may even prove necessary to introduce more resources into the operation to compensate
for the loss of efficiency resulting from work-time variation.
Line balancing
Balancing loss
Precedence diagram
Balancing work-time allocation
One of the most important design decisions in product layout is that of line balancing. In
the mortgage-processing example we have assumed that the 15 minutes of work content are
allocated equally to the four stations. This is nearly always impossible to achieve in practice
and some imbalance in the work allocation results. Inevitably this will increase the effective
cycle time of the line. If it becomes greater than the required cycle time, it may be necessary
to devote extra resources, in the shape of a further stage, to compensate for the imbalance.
The effectiveness of the line-balancing activity is measured by balancing loss. This is the time
wasted through the unequal allocation of work as a percentage of the total time invested in
processing the product or service.
Balancing techniques 5
There are a number of techniques available to help in the line-balancing task. Again, in
practice, the most useful (and most used) ‘techniques’ are the relatively simple such as the
precedence diagram. This is a representation of the ordering of the elements which compose