OPERATIONS SCHEDULING SUPPLEMENT J J-1 J …

1 OPERATIONS SCHEDULINGSUPPLEMENT JJ-1 LEARNING GOALSA fter reading this SUPPLEMENT , you shouldbe able new performance measures (beyondflow time and past due) for evaluating the decision rules (beyond FCFSand EDD) to sequence schedules for single andmultiple SUPPLEMENT focuses on operationsscheduling, which involves assigning jobs toworkstations or employees to jobs for speci-fied time periods. Effective SCHEDULING helps man-agers achieve the full potential of their supplychains. Chapter 14, OPERATIONS Planning andScheduling, covers the basics of SCHEDULING Ganttcharts, workforce SCHEDULING , two rules (FCFS andEDD) for sequencing work at a single workstation,and two commonly used performance measures(flow time and past due). Here we deepen yourunderstanding with additional performance mea-sures and priority sequencing rules, a discussion ofscheduling multiple workstations, and a discussion ofscheduling a two-station flow and the Companion Website many tools,activities, and resources designed for this JOPERATIONS SCHEDULINGS cheduling Service and ManufacturingProcessesThe SCHEDULING techniques we discuss in this SUPPLEMENT cut across the various processtypes found in services and manufacturing.

2 Many service firms are characterized by afront-office processwith high customer contact, divergent work flows, customization,and, consequently, a complex SCHEDULING environment. Often customer demands aredifficult to predict, which puts a high premium on SCHEDULING employees to handle thevaried needs of customers. At the other extreme in the service industry, a back-officeprocesshas low customer involvement, uses more line work flows, and provides stan-dardized services. Inanimate objects are processed; these processes take on the appear-ance of manufacturing processes also benefit from OPERATIONS SCHEDULING techniques. Ourdiscussion of the OPERATIONS SCHEDULING techniques in this SUPPLEMENT has applicationfor job, batch, and line processes in services as well as in manufacturing.

3 Schedules forcontinuous processes can be developed with linear programming(see SUPPLEMENT E, Linear Programming ). Although the SCHEDULING techniques in this chapter providesome structure to the selection of good schedules, many alternatives typically need to beevaluated. We begin by looking at the performance measures managers use to selectgood MeasuresWe already covered two important performance measures in Chapter 14, OperationsPlanning and SCHEDULING . Flow timeis the time a job spends in the service or manufactur-ing system, and past due(tardiness) is the amount of time by which a job missed its duedate. In this regard, a jobis the object receiving service or being manufactured. For example,a job may be a customer waiting for service at a state licensing bureau or it may be a batchof pistons waiting for a manufacturing process.

4 These two performance measures can beinsufficient, depending on the competitive priorities of a process. Additional performancemeasures follow: total amount of time required to complete a group of jobs is calledmakespan. Minimizing makespan supports the competitive priorities of cost (lowerinventory) and time (delivery speed). Total performance measure is used to measure the effectivness ofschedules for manufacturing processes. The sum ofscheduled receiptsandon-handinventoriesis thetotal total inventory supports the competitive priority of cost (inventory hold-ing costs). degree to which equipment, space, or the workforce is currently beingused, measured as the ratio of the average output rate to maximum the utilization of a process supports the competitive priority of cost (slackcapacity).

5 These performance measures often are interrelated. For example, minimizingthe average flow time tends to increase utilization. Minimizing the makespan for a groupof jobs tends to increase utilization. Understanding how flow time, makespan, past due,and utilization interact can make the selection of good schedules JobsOperations schedules are short-term plans designed to implement the sales and opera-tions plan. Often, several jobs must be processed at one or more workstations. Typically,a variety of tasks can be performed at each workstation. If schedules are not carefullyplanned to avoid bottlenecks, waiting lines may develop. For example, Figure depictsTotal inventory = Scheduled receipts for all items + On-hand inventories of all itemsMakespan = Time of completion of last job - Starting time of the first joboperations schedulingA type of SCHEDULING in which jobs areassigned to workstations or employeesare assigned to jobs for specified total amount of time required tocomplete a groupof inventoryThe sum of scheduled receipts and on-hand SCHEDULINGSUPPLEMENT JJ-3the complexity of SCHEDULING a manufacturing process.

6 When a job order is received fora part, the raw materials are collected and the batch is moved to its first operation. Thecolored arrows show that jobs follow different routes through the manufacturingprocess, depending on the product being made. At each workstation, the next job toprocess is a decision because the arrival rate of jobs at a workstation often differs fromthe processing rate of the jobs at a workstation, thereby creating a waiting line. In addi-tion, new jobs can enter the process at any time, thereby creating a dynamic environ-ment. Such complexity puts pressure on managers to develop SCHEDULING proceduresthat will handle the workload this section, we focus on SCHEDULING approaches used in two environments: (1) diver-gent flow processes and (2) line flow processes.

7 A manufacturer's operation with divergentflows is often called a job shop , which specializes in low- to medium-volume productionand utilizes jobor batchprocesses. The front officewould be the equivalent for a serviceprovider. Jobs in divergent flow processes are difficult to schedule because of the variabilityin job routings and the continual introduction of new jobs to be processed. Figure depictsa manufacturer s job shop . A manufacturer's operation with line flows is often called a flowshop, which specializes in medium- to high-volume production and utilizes lineorcontinuous flowprocesses. The back officewould be the equivalent for a service are easier to schedule because the jobs have a common flow pattern through the sys-tem.

8 Nonetheless, SCHEDULING mistakes can be costly in either Shop SequencingJust as many schedules are feasible for a specific group of jobs at a particular set of workstations,numerous methods can be used to generate schedules. They range from straightforwardmanual methods, such as manipulating Gantt charts, to sophisticated computer models fordeveloping optimal schedules. One way to generate schedules in job shops is by usingpriority sequencing rules, which allows the schedule for a workstation to evolve over a periodof time. The decision about which job to process next is made with simple priority ruleswhenever the workstation becomes available for further processing. One advantage of thismethod is that last-minute information on operating conditions can be incorporated intothe schedule as it already covered two important sequencing rules in Chapter 14, OPERATIONS Planningand SCHEDULING .

9 The first-come, first-served (FCFS)rule gives the job arriving at the workstationfirst the highest priority. The earliest due date (EDD)rule gives the job with the earliest due datebased on assigned due dates the highest priority. Such rules can be applied by a worker orjob shopA manufacturer's operation that specializesin low- to medium-volume production andutilizes job or batch shopA manufacturer's operation thatspecializes in medium- to high-volumeproduction and utilizes line or continuousflow processes. FIGURE of a Manufacturing JobShop ProcessRaw materialsShipping departmentLegend:Batch of partsWorkstationJ-4 SUPPLEMENT JOPERATIONS SCHEDULING incorporated into a computerized SCHEDULING system that generates a dispatch list of jobs andpriorities for each workstation.

10 Additional priority sequencing rules follow: Critical critical ratio (CR)is calculated by dividing the time remaining untila job s due date by the total shop time remaining for the job, which is defined as thesetup, processing, move, and expected waiting times of all remaining OPERATIONS ,including the operation being scheduled. The formula iscritical ratio (CR)A ratio that is calculated by dividing thetime remaining until a job s due date by thetotal shop time remaining for the job, whichis defined as the setup, processing, move,and expected waiting times of all remainingoperations, including the operation processing time (SPT)A priority sequencing rule that specifies thatthe job requiring the shortest processingtime is the next job to be per remaining OPERATIONS (S/RO)