2 Learning Objectives Explain the different kinds of scheduling Describe different shop loading methodsDevelop a schedule using priority rulesCalculate scheduling performance measuresDevelop a schedule for multiple workstationsDescribe the theory of constraintsDescribe scheduling for service applicationsDevelop a workforce schedule in which each employee has two consecutive days off
3 Scheduling Definitions Routing:The operations to be performed, their sequence, the work centers visited, & the time standardsBottleneck:A resource whose capacity is less than the demand placed on itDue date:When the job is supposed to be finishedSlack:The time that a job can be delayed & still finish by its due dateQueue:A waiting line
4 Scheduling: Continuous vs. Intermittent Flows Continuous (high-volume) operations are generally set so that there is little variation in the product/service or the sequence of jobs and tasks performed.Intermittent (low-volume) operations, on the other hand, have a lot of flexibility and capacity to handle varying products with differing operations in varying sequences. Most of Chapter 15 deals with intermittent operations.
5 High-Volume Operations Flow operations like automobiles, bread, gasolineHigh-volume standard items; discrete or continuous with smaller profit marginsDesigned for high efficiency and high utilizationHigh volume flow operations with fixed routingsBottlenecks are easily identifiedCommonly use line-balancing to design the process around the required tasks
6 Low-Volume Operations Low-volume, job shop operations, are designed for flexibility.Use more general purpose equipmentCustomized products with higher marginsEach product or service may have its own routing (scheduling is much more difficult)Bottlenecks move around depending upon the products being produced at any given time
7 Low-Volume Tool – Gantt Charts Developed in the early 1900’s by Henry GanttLoad charts (see below Figure 15.1)Illustrates the workload relative to the capacity of a resourceShows today’s job schedule by employee
8 Gantt Chart (continued) Progress charts:Illustrates the planned schedule compared to actual performanceBrackets show when activity is scheduled: start to finish. Design and pilot run both finished late and feedback has not started yet.
9 Scheduling Work - Work Loading Infinite loading:Ignores capacity constraints, but helps identify bottlenecks in a proposed schedule to enable proactive managementFinite loading:Allows only as much work to be assigned as can be done with available capacity – but doesn’t prepare for inevitable slippage
10 Other Scheduling Techniques Forward Scheduling – starts processing immediately when a job is receivedBackward Scheduling – begin scheduling the job’s last activity so that the job is finished on due date
12 Monitoring Work Flow Input/Output Control I/O control is a capacity-control technique used to monitor work flow at individual work centersMonitors how well available capacity is used and provides insight into process problems
13 Job Sequencing Considerations Which of several jobs should be scheduled first?Techniques are available to do short-term planning of jobs based on available capacity & prioritiesPriority rules:Decision rules to allocate the relative priority of jobs at a work centerLocal priority rules: determines priority based only on jobs at that workstationGlobal priority rules: also considers the remaining workstations a job must pass through
14 Commonly Used Priorities Rules First come, first served (FCFS)Last come, first served (LCFS)Earliest due date (EDD)Shortest processing time (SPT)Longest processing time (LPT)Critical ratio (CR):(Time until due date)/(processing time)Slack per remaining Operations (S/RO)Slack /(number of remaining operations)
16 Scheduling: n/1 Data: Evaluation: Jobs (in order of arrival) Processing time(s).Due date(s).Evaluation:Total/average flow time.Average number of jobs in system.MakespanAverage job lateness/tardiness.
17 Measuring Scheduling Performance Job flow time:Time a job is completed minus the time the job was first available for processing; avg. flow time measures responsivenessAverage # jobs in system:Measures amount of work-in-progress; avg. # measures responsivenessMakespan:The time it takes to finish a batch of jobs; measure of efficiencyJob lateness:Whether the job is completed ahead of, on, or behind schedule;Job tardiness:How long after the due date a job was completed, measures due date performance
18 Scheduling Performance Calculations Job A finishes on day 10Job B finishes on day 13Job C finishes on day 17Job D ends on day 20Calculation mean flow time:MFT= (sum job flow times)/ # of jobs= ( )/4 = 60/4 = 15 daysCalculating average number of jobs in the system:Average # Jobs =( sum job flow times)/ # days to complete batch= (60)/20 = 3 jobMakespan is the length of time to complete a batchMakespan = Completion time for Job D minus start time for Job A= 20 – 0 = 20 days
19 Performance Calculations (Cont.) Lateness and Tardiness are both measures related to customer serviceAverage tardiness is a more relevant Customer Service measurement as illustrated below
20 Comparing SPT and S/RO E done at end of day 2 A end of day 5 D at end of day 9F at end of day 14C at end of day 20B done at end of day 27
21 Comparing SPT and S/RO (cont.) B done at end of day 7A at end of day 10F at end of day 15E at end of day 17D at end of day 21C done at end of day 27
22 Sequencing Jobs through Two Work Centers –Johnson’s Rule Johnson’s Rule – a technique for minimizing makespan in a two-stage, unidirectional processStep 1 – List the jobs and the processing time for each activityStep 2 – Find the shortest activity processing time among the jobs not yet scheduledIf the shortest Processing time is for a 1st activity, schedule that job in the earliest available position in the job sequenceIf the shortest processing time is for 2nd activity, schedule that job in the last available position in the job sequenceWhen you schedule a job eliminate it from further considerationStep 3 – Repeat step 2 until you have put all activities for the job in the schedule
23 Johnson’s Rule Example: Vicki’s Office Cleaners does the annual major cleaning of university buildings. The job requires mopping (1st activity) and waxing (2nd activity) of each building. Vicki wants to minimize the time it takes her crews to finish cleaning (minimize makespan) the five buildings. She needs to finish in 20 days.
24 Scheduling Bottlenecks - OPT In the 1970’s Eli Goldratt introduced optimized production technology (OPT)OPT focused on bottlenecks for scheduling & capacity planningDefinitions:Throughput: quantity of finished goods that can be soldProcess batch: quantity produced at a resource before switching to another productTransfer batch: quantity routed at one time from one resource to the next
25 OPT Principles Balance the process rather than the flow Non-bottleneck usage is driven by some other constraint in the systemUse and activation of a resource are not the sameA hour lost at a bottleneck is lost forever, but an hour lost at a non-bottleneck is a mirageBottleneck determine throughput and inventory in systemThe transfer batch does not need to be equal to the process batchThe process batch should be variableConsider all constraints simultaneously. Lead times are the result of the schedule and are not predetermined .
26 Theory of ConstraintsTOC is an extension of OPT – theory is that a system’s output is determined by its constraintsIdentify the bottleneck(s) in the processExploit (fully utilize) the bottleneck(s)Schedule non-bottlenecks to support maximum use of bottleneck activitiesConsider adding capacity at the bottleneckContinually check for new bottlenecks
27 Scheduling for Service Organizations Demand management:Appointments & reservationsPosted availabilityDelayed services or backlogs (queues)Managing service capacity:Staff for peak demand (if cost isn’t prohibitive)Floating employees or employees on callTemporary, seasonal, or part-time employees
28 Scheduling Workers in Service Operations Forecast customer demand.Convert customer demand into worker requirements.Convert worker requirements into daily work schedules.Convert daily work schedules into weekly work schedules.
29 Developing a Workforce Schedule: Tibrewala, Philippe, and Brown developed a technique for scheduling a seven day operation giving each employee two consecutive days off. This example shows how a staff of six people can be scheduled.Step 1 – Find out the minimum number of employees needed for each day of the weekStep 2 – Given the above requirements, calculate the number of employees needed for each pair of consecutive daysStep 3 - Find the pair of days with the lowest total needed
30 Workforce Scheduling (cont.) Step 4 – Update the number of employees you still need to schedule for each dayStep 5 – Using the updated staffing needs, repeat steps 2 through 4 until you have satisfied all needs
33 Final ScheduleThis technique gives a work schedule for each employee to satisfy minimum daily staffing requirementsNext step is to replace numbers with employee namesManager can give senior employees first choice and proceed until all employees have a schedule
34 Chapter 15 HighlightsScheduling techniques depend on volume. High volume is typically done through line design and balancing. Low volume uses priority rules along with visual techniques like Gantt charts.Shop loading can assume infinite or finite loading which is constrained by predetermined capacity. Loading can be done by using forward or backward scheduling.Scheduling decisions use common priority rules like SPT, EDD, FCFS, and S/RO. Priority rules need to support organizational objectives.Performance measures like mean flow time, job lateness, job tardiness, makespan, and the average number of jobs in the system measure the effectiveness of schedules.
35 Chapter 15 HighlightsJohnson’s Rule is a effective technique for minimizing makespan when successive workstations are needed to complete the process.OPT principles can be used to schedule bottlenecks. TOC expands OPT into a continuous improvement philosophy.Service organizations use different techniques such as appointments, reservations, and posted schedules for use of service capacity.Techniques exist for workforce scheduling when a company uses full time employees, operates 7 days each week, and gives its employees 2 consecutive days off.
36 Homework HintsProblem 15.5: Use the SPT priority rule to schedule, then measure performance.Problem 15.6: Use the EDD priority rule to schedule, then measure performance.Problem Use the FCFS priority rule to schedule, then measure performance. Add c. to compare results of #5, #6, and #8.Problem 15.15: Use Johnson’s rule. Determine performance measures, but don’t compare to #13 and #14.Problem 15.20: Assume all employees are full-time (e.g., working 5 days/week).