Ch 15 Short Term Scheduling

Ch 15 Short Term Scheduling - Operations Management Chapter...

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Unformatted text preview: Operations Management Chapter 15 – Chapter Short-Term Scheduling Short-Term 15 – 1 Outline Global Company Profile: Global Delta Air Lines Delta The Strategic Importance of ShortTerm Scheduling Scheduling Issues Forward and Backward Scheduling Scheduling Criteria 15 – 2 Outline – Continued Scheduling Process-Focused Scheduling Facilities Facilities Loading Jobs Input-Output Control Gantt Charts Assignment Method 15 – 3 Outline – Continued Sequencing Jobs Priority Rules for Dispatching Jobs Critical Ratio Sequencing N Jobs on Two Sequencing Machines: Johnson’s Rule Machines: Limitations of Rule-Based Limitations Dispatching Systems Dispatching Finite Capacity Scheduling (FCS) 15 – 4 Outline – Continued Theory of Constraints Bottlenecks Drum, Buffer, Rope Scheduling Repetitive Facilities Scheduling Services Scheduling Service Employees with Scheduling Cyclical Scheduling Cyclical 15 – 5 Learning Objectives When you complete this chapter you should be able to: 1. Explain the relationship between shortterm scheduling, capacity planning, term aggregate planning, and a master schedule schedule 2. Draw Gantt loading and scheduling Draw charts charts 3. Apply the assignment method for Apply loading jobs loading 15 – 6 Learning Objectives When you complete this chapter you should be able to: 1. Name and describe each of the priority Name sequencing rules sequencing 2. Use Johnson’s rule 3. Define finite capacity scheduling 4. List the steps in the theory of List constraints constraints 5. Use the cyclical scheduling technique 15 – 7 Delta Airlines About 10% of Delta’s flights are About disrupted per year, half because of weather weather Cost is $440 million in lost revenue, Cost overtime pay, food and lodging vouchers vouchers The $33 million Operations Control The Center adjusts to changes and keeps flights flowing flights Saves Delta $35 million per year 15 – 8 Strategic Importance of Short-Term Scheduling Effective and efficient scheduling Effective can be a competitive advantage can Faster movement of goods through a Faster facility means better use of assets and lower costs and Additional capacity resulting from Additional faster throughput improves customer service through faster delivery service Good schedules result in more Good dependable deliveries dependable 15 – 9 Scheduling Issues Scheduling deals with the timing of Scheduling operations operations The task is the allocation and The prioritization of demand prioritization Significant issues are The type of scheduling, forward or The backward backward The criteria for priorities 15 – 10 Scheduling Decisions Organization Arnold Palmer Hospital Managers Must Schedule the Following Operating room use Patient admissions Nursing, security, maintenance staffs Outpatient treatments Classrooms and audiovisual equipment Student and instructor schedules Graduate and undergraduate courses Production of goods Purchases of materials Workers Chef, waiters, bartenders Delivery of fresh foods Entertainers Opening of dining areas Maintenance of aircraft Departure timetables Flight crews, catering, gate, ticketing personnel 15 – 11 University of Missouri Lockheed Martin factory Hard Rock Cafe Delta Air Lines Table 15.1 Scheduling Flow Figure 15.1 15 – 12 Forward and Backward Scheduling Forward scheduling starts as soon Forward as the requirements are known as Produces a feasible schedule Produces though it may not meet due dates though Frequently results in Frequently buildup of work-inbuildup process inventory Now Due Due Date Date 15 – 13 Forward and Backward Scheduling Backward scheduling begins with Backward the due date and schedules the final operation first operation Schedule is produced by working Schedule backwards though the processes backwards Resources may not Resources be available to accomplish the Due Due Now Date Date schedule schedule 15 – 14 Forward and Backward Scheduling Backward scheduling begins with Backward e the due date and schedules the final es ar off h proac tradeoperation tfirstap elop a le and operation hese v u Often ed to de ched s bin Schedule is fproducedteby working sible da s Schedulen a ea due com e r betwe custome backwards though the processes backwards Resources may not Resources be available to accomplish the Due Due Now Date Date schedule schedule 15 – 15 Different Processes/ Different Approaches Process-focused facilities Work cells Forward-looking schedules MRP due dates Finite capacity scheduling Forward-looking schedules MRP due dates Detailed schedule done using work cell priority rules Forward-looking schedule with a balanced line Pull techniques for scheduling Forward-looking schedule with stable demand and fixed capacity Capacity, set-up, and run times known Capacity limited by long-term capital investment 15 – 16 Repetitive facilities Product-focused facilities Table 15.2 Scheduling Criteria 1. Minimize completion time 2. Maximize utilization of facilities 3. Minimize work-in-process (WIP) Minimize inventory inventory 4. Minimize customer waiting time Optimize the use of resources so Optimize that production objectives are met that 15 – 17 Scheduling ProcessFocused Facilities 1. Schedule incoming orders without Schedule violating capacity constraints violating 2. Check availability of tools and materials Check before releasing an order before 3. Establish due dates for each job and Establish check progress check 4. Check work in progress 5. Provide feedback 6. Provide work efficiency statistics and Provide monitor times monitor 15 – 18 Planning and Control Files Planning Files 1. An item master file contains information about An each component each 2. A routing file indicates each component’s flow routing through the shop through 3. A work-center master file contains information work-center about the work center about Control Files Track the actual progress made against Track the plan the 15 – 19 Loading Jobs Assign jobs so that costs, idle Assign time, or completion time are minimized minimized Two forms of loading Capacity oriented Assigning specific jobs to work Assigning centers centers 15 – 20 Input-Output Control Identifies overloading and Identifies underloading conditions underloading Prompts managerial action to Prompts resolve scheduling problems resolve Can be maintained using ConWIP Can cards that control the scheduling of batches of 15 – 21 Input-Output Control Example Work Center DNC Milling (in standard hours) Week Ending Planned Input Actual Input Cumulative Deviation Planned Output Actual Output Cumulative Deviation Cumulative Change in Backlog 6/6 280 270 –10 320 270 –50 0 6/13 280 250 –40 320 270 –100 –20 6/20 280 280 –40 320 270 –150 –10 6/27 280 285 –35 320 270 –200 +5 7/4 280 280 7/11 Figure 15.2 15 – 22 Input-Output Control Example Work Center DNC Milling (in standard hours) Week Ending Planned Input Actual Input Cumulative Deviation Planned Output 6/6 280 270 –10 320 270 –50 0 6/13 280 250 –40 320 270 –100 –20 6/20 280 280 –40 320 270 –150 –10 6/27 280 285 –35 320 270 250 input, +5 7/4 280 280 7/11 Explanation: 270 input, Actual Output 270 output implies 0 change Cumulative Deviation Cumulative Change in Backlog Explanation: 270 output implies –200 –20 change Figure 15.2 15 – 23 Input-Output Control Example Options available to operations Options personnel include: personnel 1. Correcting performances 2. Increasing capacity 3. Increasing or reducing input to Increasing the work center the 15 – 24 Gantt Charts Load chart shows the loading and Load idle times of departments, machines, or facilities or Displays relative workloads over Displays time time Schedule chart monitors jobs in Schedule process process All Gantt charts need to be updated All frequently to account for changes frequently 15 – 25 Gantt Load Chart Example Work Center Day Monday Job 349 Job 349 Job 408 Job 295 Job 408 Tuesday Wednesday Thursday Job 350 Job 408 Job 349 Job 349 Friday Metalworks Mechanical Electronics Painting Processing Unscheduled Center not available Figure 15.3 15 – 26 Gantt Schedule Chart Example Job A N o w Maintenance Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 8 Start of an Start activity activity End of an End activity activity Scheduled Scheduled activity time allowed allowed Actual work Actual progress progress B C Figure 15.4 Nonproduction Nonproduction time time 15 – 27 Assignment Method A special class of linear special programming models that assign tasks or jobs to resources tasks Objective is to minimize cost or Objective time time Only one job (or worker) is Only assigned to one machine (or project) project) 15 – 28 Assignment Method Build a table of costs or time Build associated with particular assignments assignments Typesetter B $14 $10 $12 Job R-34 S-66 T-50 A $11 $8 $9 C $6 $11 $7 15 – 29 Assignment Method 1. Create zero opportunity costs by Create repeatedly subtracting the lowest costs from each row and column from 2. Draw the minimum number of vertical Draw and horizontal lines necessary to cover all the zeros in the table. If the number of lines equals either the number of rows or the number of columns, proceed to step 4. Otherwise proceed to step 3. step 15 – 30 Assignment Method 1. Subtract the smallest number not Subtract covered by a line from all other uncovered numbers. Add the same number to any number at the intersection of two lines. Return to step 2. step 2. Optimal assignments are at zero Optimal locations in the table. Select one, draw lines through the row and column involved, and continue to the next assignment. assignment. 15 – 31 Assignment Example Typesetter A $11 $8 $9 B $14 $10 $12 C $6 $11 $7 Job R-34 S-66 T-50 Step 1a - Rows Typesetter Step 1b - Columns C $ $ $ 0 3 0 Typesetter A $ $ $ 5 0 2 B $ $ $ 8 2 5 A $ $ $ 5 0 2 B $ $ $ 6 0 3 C $ $ $ 0 3 0 15 – 32 Job R-34 S-66 T-50 Job R-34 S-66 T-50 Assignment Example Step 2 - Lines Typesetter A $ $ $ 5 0 2 B $ $ $ 6 0 3 C $ $ $ 0 3 0 Job R-34 S-66 T-50 The smallest uncovered The number is 2 so this is subtracted from all other uncovered numbers and added to numbers at the intersection of lines intersection Step 3 - Subtraction Typesetter Because only two lines Because are needed to cover all the zeros, the solution is not optimal is A $ $ $ 3 0 0 B $ $ $ 4 0 1 C $ $ $ 0 5 0 15 – 33 Job R-34 S-66 T-50 Assignment Example Step 2 - Lines Typesetter A $ $ $ 3 0 0 B $ $ $ 4 0 1 C $ $ $ 0 5 0 Job R-34 S-66 T-50 Start by assigning R-34 to Start worker C as this is the only possible assignment for worker C. Job T-50 must Job worker go to worker A as worker C is already assigned. This leaves S-66 for worker B. leaves Step 4 - Assignments Typesetter Because three lines are Because needed, the solution is optimal and assignments can be made made A $ $ $ 3 0 0 B $ $ $ 4 0 1 C $ $ $ 0 5 0 15 – 34 Job R-34 S-66 T-50 Assignment Example Step 4 - Assignments Typesetter A $11 $8 $9 B $14 $10 $12 C $6 $11 $7 Typesetter A $ $ $ 3 0 0 B $ $ $ 4 0 1 C $0 $5 $0 Job R-34 S-66 T-50 Job R-34 S-66 T-50 From the original cost table Minimum cost = $6 + $10 + $9 = $25 15 – 35 Sequencing Jobs Specifies the order in which jobs Specifies should be performed at work centers should Priority rules are used to dispatch or Priority sequence jobs sequence FCFS: First come, first served SPT: Shortest processing time EDD: Earliest due date LPT: Longest processing time 15 – 36 Sequencing Example Apply the four popular sequencing rules Apply to these five jobs to Job A B C D E Job Work (Processing) Time (Days) 6 2 8 3 9 Job Due Date (Days) 8 6 18 15 23 15 – 37 Sequencing Example FCFS: Sequence A-B-C-D-E Job Sequence A B C D E Job Work (Processing) Time 6 2 8 3 9 28 Flow Time 6 8 16 19 28 77 Job Due Date 8 6 18 15 23 Job Lateness 0 2 0 4 5 11 15 – 38 Sequencing Example FCFS: Sequence A-B-C-D-E Job Work (Processing) Time A 6 Average completion time = B C Utilization = 2 8 Job Sequence Flow Job Due Job Sum Time of total flow time Date Lateness Number of jobs 6 8 0 Total job work time = 77/5 = 15.4 days Sum of total flow time 8 6 2 = 2 flow time 16 18 Sum of total8/77 = 36.4% Total job work time 19 15 77 Number of jobs 0 4 5 D 3 Average number of jobs in the system E 9 28 Total late days 23 28 Average job lateness = 11 = 11/5 = 2.2 days 15 – 39 Sequencing Example SPT: Sequence B-D-A-C-E Job Sequence B D A C E Job Work (Processing) Time 2 3 6 8 9 28 Flow Time 2 5 11 19 28 65 Job Due Date 6 15 8 18 23 Job Lateness 0 0 3 1 5 9 15 – 40 Sequencing Example SPT: Sequence B-D-A-C-E Job Work (Processing) Time B 2 Average completion time = D A Utilization = 3 6 Job Sequence Flow Job Due Job Sum Time of total flow time Date Lateness Number of jobs 2 6 0 Total job work time = 65/5 = 13 days Sum of total flow time 5 15 0 = 2 flow time 11 8 Sum of total8/65 = 43.1% Total job work time 19 18 65 Number of jobs 3 1 5 C 8 Average number of jobs in the system E 9 28 Total late days 23 28 Average job lateness = 9 = 9/5 = 1.8 days 15 – 41 Sequencing Example EDD: Sequence B-A-D-C-E Job Sequence B A D C E Job Work (Processing) Time 2 6 3 8 9 28 Flow Time 2 8 11 19 28 68 Job Due Date 6 8 15 18 23 Job Lateness 0 0 0 1 5 6 15 – 42 Sequencing Example EDD: Sequence B-A-D-C-E Job Work (Processing) Time B 2 Average completion time = A D Utilization = 6 3 Job Sequence Flow Job Due Job Sum Time of total flow time Date Lateness Number of jobs 2 6 0 Total job work time = 68/5 = 13.6 days Sum of total flow time 8 8 0 = 2 flow time 11 15 Sum of total8/68 = 41.2% Total job work time 19 18 68 Number of jobs 0 1 5 C 8 Average number of jobs in the system E 9 28 Total late days 23 28 Average job lateness = 6 = 6/5 = 1.2 days 15 – 43 Sequencing Example LPT: Sequence E-C-A-D-B Job Sequence E C A D B Job Work (Processing) Time 9 8 6 3 2 28 Flow Time 9 17 23 26 28 103 Job Due Date 23 18 8 15 6 Job Lateness 0 0 15 11 22 48 15 – 44 Sequencing Example LPT: Sequence E-C-A-D-B Job Work (Processing) Time E 9 Average completion time = C AUtilization = 8 6 Job Sequence Flow Job Due Job Sum Time of total flow time Date Lateness Number of jobs 9 23 0 = Total job work time 103/5 = 20.6 days Sum of total flow time 17 18 0 = 28/103 = 27.2% 15 23 8 Sum of total flow time Total job work time 26 15 11 22 103 Number of jobs D 3 Average number of jobs in the system days B 2 28 Total late6 28 Average job lateness = 48 = 48/5 = 9.6 days 15 – 45 Sequencing Example Summary of Rules Rule FCFS SPT EDD LPT Average Completion Time (Days) 15.4 13.0 13.6 20.6 Utilization (%) 36.4 43.1 41.2 27.2 Average Number of Jobs in System 2.75 2.32 2.43 3.68 Average Lateness (Days) 2.2 1.8 1.2 9.6 15 – 46 Comparison of Sequencing Rules No one sequencing rule excels on all criteria SPT does well on minimizing flow time and SPT number of jobs in the system number But SPT moves long jobs to But the end which may result in dissatisfied customers in FCFS does not do especially FCFS well (or poorly) on any criteria but is perceived as fair by customers as EDD minimizes lateness 15 – 47 Critical Ratio (CR) An index number found by dividing the An time remaining until the due date by the work time remaining on the job work Jobs with low critical ratios are Jobs scheduled ahead of jobs with higher critical ratios critical Performs well on average job lateness Performs criteria criteria Time remaining Due date - Today’s date CR = = Workdays remaining Work (lead) time remaining 15 – 48 Critical Ratio Example Currently Day 25 Currently 25 Job Due Date Workdays Remaining Critical Ratio Priority Order A B C 30 28 27 4 5 2 (30 - 25)/4 = 1.25 (28 - 25)/5 = .60 (27 - 25)/2 = 1.00 3 1 2 With CR < 1, Job B is late. Job C is just on schedule With and Job A has some slack time. and 15 – 49 Critical Ratio Technique 1. Helps determine the status of specific Helps jobs jobs 2. Establishes relative priorities among Establishes jobs on a common basis jobs 3. Relates both stock and make-to-order Relates jobs on a common basis jobs 4. Adjusts priorities automatically for Adjusts changes in both demand and job progress progress 5. Dynamically tracks job progress 15 – 50 Sequencing N Jobs on Two Machines: Johnson’s Rule Works with two or more jobs that Works pass through the same two machines or work centers machines Minimizes total production time and Minimizes idle time idle 15 – 51 Johnson’s Rule 1. List all jobs and times for each work List center center 2. Choose the job with the shortest activity Choose time. If that time is in the first work center, schedule the job first. If it is in the second work center, schedule the job last. work 3. Once a job is scheduled, it is eliminated Once from the list 4. Repeat steps 2 and 3 working toward the 4. Repeat center of the sequence center 15 – 52 Johnson’s Rule Example Job A B C D E Work Center 1 (Drill Press) 5 3 8 10 7 Work Center 2 (Lathe) 2 6 4 7 12 15 – 53 Johnson’s Rule Example Job A B C D E Work Center 1 (Drill Press) 5 3 8 10 7 Work Center 2 (Lathe) 2 6 4 7 12 BEDCA 15 – 54 Johnson’s Rule Example Job A B C D E Time 0 3 Work Center 1 (Drill Press) 5 3 8 10 7 10 Work Center 2 (Lathe) 2 6 4 7 12 20 28 33 BEDCA WC 1 WC 2 B E D C A 15 – 55 Johnson’s Rule Example Job A B C D E Time 0 3 Work Center 1 (Drill Press) 5 3 8 10 7 10 Work Center 2 (Lathe) 2 6 4 7 12 20 28 33 BEDCA WC 1 WC 2 B E B 3 5 D E 7 9 10 11 12 13 C D 17 19 21 22 2325 27 A C D A CA 15 – 56 Time 0 1 35 29 31 33 B E Limitations of Rule-Based Dispatching Systems 1. Scheduling is dynamic and rules Scheduling need to be revised to adjust to changes changes 2. Rules do not look upstream or Rules downstream downstream 3. Rules do not look beyond due Rules dates dates 15 – 57 Finite Capacity Scheduling Overcomes disadvantages of rule-based Overcomes systems by providing an interactive, computer-based graphical system computer-based May include rules and expert systems or May simulation to allow real-time response to system changes system Initial data often from an MRP system FCS allows the balancing of delivery FCS needs and efficiency needs 15 – 58 Finite Capacity Scheduling MRP Data • Master schedule • BOM • Inventory Priority rules • Expert systems • Simulation models Interactive Finite Capacity Scheduling • Routing files • Work center information Tooling and other resources Setups and run time Figure 15.5 15 – 59 Finite Capacity Scheduling 15 – 60 Theory of Constraints Throughput is the number of units Throughput processed through the facility and sold processed TOC deals with the limits an organization TOC faces in achieving its goals faces 1. 2. 3. 4. Identify the constraints Develop a plan for overcoming the constraints Focus resources on accomplishing the plan Reduce the effects of constraints by offloading work or increasing capacity 5. Once successful, return to step 1 and identify Once new constraints new 15 – 61 Bottlenecks Bottleneck work centers are constraints Bottleneck that limit output that Common occurrence due to frequent changes Management techniques include: Increasing the capacity of the constraint Cross-trained employees and maintenance Alternative routings, procedures, or Alternative subcontractors subcontractors Moving inspection and test Scheduling throughput to match bottleneck Scheduling capacity capacity 15 – 62 Drum, Buffer, Rope The drum is the beat of the system and The provides the schedule or pace of production production The buffer is the inventory necessary to The keep constraints operating at capacity keep The rope provides the synchronization The necessary to pull units through the system system 15 – 63 Scheduling Repetitive Facilities Level material use can help Level repetitive facilities Better satisfy customer Better demand demand Lower inventory Lower investment investment Reduce batch size Better utilize equipment Better and facilities and 15 – 64 Scheduling Repetitive Facilities Advantages include: 1. Lower inventory levels 2. Faster product throughput 3. Improved component quality 4. Reduced floor-space requirements 5. Improved communications 6. Smoother production process 15 – 65 Scheduling Services Service systems differ from manufacturing Manufacturing Schedules machines and materials Inventories used to smooth demand Machine-intensive and demand may be smooth Scheduling may be bound by union contracts Few social or behavioral issues Services Schedule staff Seldom maintain inventories Labor-intensive and demand may be variable Legal issues may constrain flexible scheduling Social and behavioral issues may be quite important 15 – 66 Scheduling Services Hospitals have complex scheduling Hospitals system to handle complex processes and material requirements and Banks use a cross-trained and flexible Banks workforce and part-time workers workforce Retail stores use scheduling Retail optimization systems that track sales, transactions, and customer traffic to create work schedules in less time and with improved customer satisfaction with 15 – 67 Scheduling Services Airlines must meet complex FAA and Airlines union regulations and often use linear programming to develop optimal schedules schedules 24/7 operations like police/fire 24/7 departments, emergency hot lines, and mail order businesses use flexible workers and variable schedules, often created using computerized systems created 15 – 68 Demand Management Appointment or reservation Appointment systems systems FCFS sequencing rules Discounts or other promotional Discounts schemes schemes When demand management is not When feasible, managing capacity through staffing flexibility may be used used 15 – 69 Scheduling Service Employees With Cyclical Scheduling Objective is to meet staffing Objective requirements with the minimum number of workers number Schedules need to be smooth and Schedules keep personnel happy keep Many techniques exist from simple Many algorithms to complex linear programming solutions programming 15 – 70 Cyclical Scheduling Example 1. Determine the staffing requirements 2. Identify two consecutive days with the Identify lowest total requirements and assign these as days off these 3. Make a new set of requirements Make subtracting the days worked by the first employee employee 4. Apply step 2 to the new row Apply 5. Repeat steps 3 and 4 until all 5. Repeat requirements have been met requirements 15 – 71 Cyclical Scheduling Example M Employee 1 5 T 5 W 6 T 5 F 4 S 3 S 3 Capacity (Employees) Excess Capacity 15 – 72 Cyclical Scheduling Example M Employee 1 Employee 2 5 4 T 5 4 W 6 5 T 5 4 F 4 3 S 3 3 S 3 3 Capacity (Employees) Excess Capacity 15 – 73 Cyclical Scheduling Example M Employee 1 Employee 2 Employee 3 5 4 3 T 5 4 3 W 6 5 4 T 5 4 3 F 4 3 2 S 3 3 3 S 3 3 3 Capacity (Employees) Excess Capacity 15 – 74 Cyclical Scheduling Example M Employee 1 Employee 2 Employee 3 Employee 4 5 4 3 2 T 5 4 3 2 W 6 5 4 3 T 5 4 3 2 F 4 3 2 2 S 3 3 3 3 S 3 3 3 2 Capacity (Employees) Excess Capacity 15 – 75 Cyclical Scheduling Example M Employee 1 Employee 2 Employee 3 Employee 4 Employee 5 5 4 3 2 1 T 5 4 3 2 1 W 6 5 4 3 2 T 5 4 3 2 2 F 4 3 2 2 2 S 3 3 3 3 2 S 3 3 3 2 1 Capacity (Employees) Excess Capacity 15 – 76 Cyclical Scheduling Example M Employee 1 Employee 2 Employee 3 Employee 4 Employee 5 Employee 6 5 4 3 2 1 1 T 5 4 3 2 1 1 W 6 5 4 3 2 1 T 5 4 3 2 2 1 F 4 3 2 2 2 1 S 3 3 3 3 2 1 S 3 3 3 2 1 0 Capacity (Employees) Excess Capacity 15 – 77 Cyclical Scheduling Example M Employee 1 Employee 2 Employee 3 Employee 4 Employee 5 Employee 6 Employee 7 5 4 3 2 1 1 T 5 4 3 2 1 1 W 6 5 4 3 2 1 T 5 4 3 2 2 1 F 4 3 2 2 2 1 S 3 3 3 3 2 1 1 S 3 3 3 2 1 0 Capacity (Employees) Excess Capacity 15 – 78 Cyclical Scheduling Example M Employee 1 Employee 2 Employee 3 Employee 4 Employee 5 Employee 6 Employee 7 5 4 3 2 1 1 T 5 4 3 2 1 1 W 6 5 4 3 2 1 T 5 4 3 2 2 1 F 4 3 2 2 2 1 S 3 3 3 3 2 1 1 S 3 3 3 2 1 0 Capacity (Employees) Excess Capacity 5 0 5 0 6 0 5 0 4 0 3 1 3 0 15 – 79 ...
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