Page 210 - Orlicky's Material Requirements Planning

CHAPTER 10 A New Way of Looking at Things 189

ority is determined by the ranking of a group of jobs. Absolute priority is given by the
relation of a job to its date of need. This is reflected in the right-hand view of the blocks
in Figure 10-6, which shows that the active queue consists, in this case, of only one-half
the jobs.
The conventional notion of queue control, represented by Figure 10-7, must be
reconsidered once priorities are taken into account. The traditional theoretical approach
to this problem is to measure a queue at a work center over a period of time (e.g., mini-
mum 60 and maximum 100 standard hours) and to remove its fixed portion (60 standard
hours) through overtime, subcontracting, and so on. The controlled queue then consists
of the variable portion that fluctuates between zero and its upper limit (0 to 40 standard
hours). This is the minimum queue required to prevent running out of work.
In reality, it would be foolhardy to assume that standard hours of work adequately
describe a queue. The units of work are not necessarily homogeneous and interchange-
able, as has been demonstrated in previous examples. If the fixed portion of the queue
were to be worked off, it certainly would be the jobs with the highest relative priority;
that is, the queue would be reduced from the top rather than from the bottom, which is
shown in Figure 10-8. When looked at this way, this entire approach to the problem
proves nonsensical because what is left at the work center are the dormant and dead por-
tions of the queue.
Important to note is that this queue analysis has relevance only at the bottleneck or
constraint in the plant. Having the nonbottlenecks run out of work has zero impact on the
overall throughput of the plant. Running the bottleneck out of work never can be recov-
ered, and the throughput is lost for the entire plant.

FIGURE 10-7 Maximum
Queue control:
conventional view.

Fluctuation

Minimum

Fixed: Eliminate