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1 4. Flow-Time Analysis Inventory adversely affects all competing edges (P/Q/V/D) Has cost Physical carrying costs Financial costs Causes obsolescence Due to market changes Due to technology changes Leads to poor quality Feed back loop is long Hides problems Unreliable suppliers. High defect rate, tools changeover times, machine breakdowns. Causes long flow time Lean Operations: The Real Cost of Inventory

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2 4. Flow-Time Analysis Flow-Time Analysis Competitive Advantages of a shorter flow time Shorter response time (both in production and product design) Delayed differentiation (Postponement) Lower inventory costs (due to Little’s Law)

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3 4. Flow-Time Analysis Direct Methods of Measuring Flow Time Direct Method Observe a process over an extended, specified period of time Randomly sample flow units over the observation period Measure the flow time for each flow unit from entry to exit Compute the average of flow times measured Example During a given month, a sample of 50 applications was taken The average flow time = 20.85 working days

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4 4. Flow-Time Analysis Indirect Methods of Measuring Flow Time Indirect Method Count the number of units produced over an extended period of time R = number of units produced / duration of time period Count the number of units of inventory at random points during the time period, and compute the average inventory (I). Compute T =I/R Example 200 applications processed during a given month, 20 days of operation during the given month. Throughput for the sample is R=200/20 = 10 applications per day. After counting the number of applications at 4 random points during the month, the average inventory (I) was 215. T=I/R T = 215/10 flow time is 21.5 days

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5 4. Flow-Time Analysis Process = network of activities performed by resources 1. Flow unit: The unit of analysis– Level of detail 2-3. Network of Activities & Storage/Buffers Define Process Boundaries Activities with activity times Buffers with waiting flow times Routes: precedence relationships (solid lines) with throughputs 4. Resources & Allocation - Who does what? 5. Information Structure & flow (dashed lines) Key for capacity analysis Process Flow Chart

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6 4. Flow-Time Analysis Flowcharting Activity Buffer Physical flow Event Information flow -See the total process; a systems view -Define flow units and process boundaries -Include only the key steps -Clarify the level of detail needed. Processes can be broken down into sub-activities. On the other hand, cascading allows several activities to be combined in a single sub-process -Depicts resources required to carry out activities -Identify the processes that need attention (weak points) Decision

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7 4. Flow-Time Analysis Wondershed Inc: Narrative Representation Required activities : 1.Separate the roof and the base sheets 2. Punch the base 3. Punch the roof 4. Form the base 4. Form the roof 6. Sub-assemble the base 7. Assemble 8. Inspect

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8 4. Flow-Time Analysis End Wondershed Inc. : Schematic Representation Start Separate Sheet Punch base Punch roof Form base Form roof Sub assemble base Inspect Assemble Start Activity A Activity B Activity C End

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9 4. Flow-Time Analysis Theoretical Flow Time Theoretical Flow Time: The minimum amount of time required for processing a typical flow unit without any waiting Entry Buffer Activity Exit Activity Time: The time required by a typical flow unit to complete the activity once. Unless stated otherwise, we assume that an activity can not be broken down into smaller activities that can be executed in parallel. What does that mean???

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10 4. Flow-Time Analysis Critical Path Theoretical flow time of each path in the process flowchart = sum of the activity times of all activities on that path Critical Path: the longest path Critical Activities: activities that lie on the critical path The theoretical flow time of the process = the time of the theoretical critical path Activity Time 2Activity Time 1Activity Time 3 Activity Time 5Activity Time 4Activity Time 6 Sequential Activities Parallel Activities How many Paths? Activity Time 2Activity Time 1Activity Time 3 Activity Time 2Activity Time 1Activity Time 3

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11 4. Flow-Time Analysis Wondershed Inc. Theoretical Flow Time Path 1 (roof) Start 1 3 5 7 8 End = 80 minutes Path 2 (base) Start 1 2 4 6 7 8 End = 90 minutes Activity Times at Wonder Shed Inc. Activity Activity Time (minutes) 1Separate10 2Punch the base25 3Punch the roof20 4Form the base5 5Form the roof10 6Subassemble the base10 7Assemble10 8 Inspect 30

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12 4. Flow-Time Analysis Average Wait Time, Average Flow Time Average waiting time for roof T = I / R = 80 / 16.5 = 4.85 hours or 291 minutes Average waiting time for base T = I / R = 90 / 16.5 = 5.45 hours or 327 minutes Theoretical flow times Roof 80 minutes Base 90 minutes Flow Times Roof 291 + 80 = 371 minutes Base 327 + 90 = 417 minutes 16.5 sheds average flow rate (R) per hour based on empirical data

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13 4. Flow-Time Analysis Value-adding vs. Non-value-adding activities Value-adding activities are those activities that increase the value of a flow units Example: assembly, surgery, piloting an airplane, etc. Non value adding activities are activities that do not directly increase the value of a flow unit. Re-work Transport Inspection Waiting Flow time = Value-adding times + Non-value-adding times

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14 4. Flow-Time Analysis Activity Time, Number of Visits and Work Content Activity time is time required by a typical flow unit to complete the activity once. Sometimes we may need to repeat an activity, i.e. the number of visits is not necessarily 1. Work content = Activity time × Average number of visits Theoretical flow time is computed based on work content not based on Activity time

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15 4. Flow-Time Analysis Activity Time vs. Work Content

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16 4. Flow-Time Analysis Theoretical Flow Time vs. Flow Time Theoretical flow time = 90 Flow time = 432 Flow Time Efficiency = Theoretical Flow Time / Flow Time Flow Time Efficiency = 105/432 = 24%

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17 4. Flow-Time Analysis Flow Time Efficiencies in White Collar Processes

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18 4. Flow-Time Analysis Reducing Theoretical Flow Time: Shorten the Length of Every Critical Path by 3 Basic Approaches Eliminate: Reducing the work content of critical activities Work smarter: Eliminate non-value-adding aspects of the activity: Work Faster: Increase the speed at which the activity is performed, e.g. Better Methods, Training, Advanced Technology, Better Management. Work right in the first time: Reduce repetitions of the activity Work in parallel: Moving work off the critical path Move work off the critical path to a noncritical activity Move work off the critical path to the outer loop (pre- or post-processing) Select: Modify the product mix Make more of a product that requires less processing time reduce overall flow time (Given market condition)

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19 4. Flow-Time Analysis Levers for Managing Theoretical Flow Time - Move work to the outer loop (pre-meeting “homework”) - Work in parallel (professionals work simultaneously) - Elimination of non-value-adding work (only two meetings)

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20 4. Flow-Time Analysis Critical Path Method: The Network A1 A3 A4 A6 A5 A2 4 3 6 4 2 3 E S Find the Critical Path.

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21 4. Flow-Time Analysis Critical Path Method: Paths A1A3A4A6A5A2 4 3 6 4 2 3 ES 10 Critical Path is the longest Path 118 How many path?

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22 4. Flow-Time Analysis Forward Path; Earliest Starts A1 A3 A4 A6 A5 A2 4 3 6 4 2 3 E S 0 0 0 0 0 0 4 3 3 4 4 3 4 4 3 4 410 8 5 5 5 88 8 11

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23 4. Flow-Time Analysis 10 30 20 Max = 30 5 35 Forward Path 35

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24 4. Flow-Time Analysis Backward Path; Latest Starts A1 A3 A4 A6 A5 A2 4 3 6 4 2 3 E S 0 0 0 0 0 0 4 3 3 4 4 3 4 4 3 4 410 8 5 5 5 88 8 11 5 55 8 8 8 8 8 8 8 4 6 4 4 6 6 6 33 3 40 0 0

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25 4. Flow-Time Analysis 30 Min = 35 5 35 45 30 Backward Path

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26 4. Flow-Time Analysis Activity Slack Slack, or float: The amount of time a noncritical task can be delayed without delaying the project Slack—LFT – EFT or LST – EST EST—Earliest Start Time; Largest EFT of all predecessors EFT—Earliest Finish Time; EST + duration for this task LFT—Latest Finish Time; Smallest LST of following tasks LST—Latest Start Time; LFT – duration for this task 5/23/2015Ardavan Asef-Vaziri5-1-26

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27 4. Flow-Time Analysis Critical Path, Slacks A1A3A4A6A5A2 4 3 6 4 2 3 ES 0 0 4 3 3 4 410 8 5 8 11 5 8 8 8 4 6 6 3 40

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28 4. Flow-Time Analysis Theoretical Critical Path vs. Critical Path A1 A3 A4 A6 A5 A2 4 3 6 4 2 3 ES W1 W2 W4 W3 W6 W7 W5 W9 W8 The time of the critical path differs from the time of the theoretical critical path. Why? The critical path itself also may differ from the theoretical critical path. Why?

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29 4. Flow-Time Analysis Practice: Draw the Network of Activities Task Pred. Dur. a -- 4 g c,d 1 b -- 3 h e 4 c a 3 i f 5 d a 2 j e,g 6 e b 6 k h,i 1 f b 4 For each task, compute ES, EF, LF, LS, slack 5/23/2015Ardavan Asef-Vaziri5-1-29

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30 4. Flow-Time Analysis Practice: Compute the Theoretical Flow Time Start Finish a=4 slack= b=3 slack= c=3 slack= d=2 slack= e=6 slack= f=4 slack= g=1 slack= h=4 slack= i=5 slack= j=6 slack= k=1 slack= Task=dur slack=xxx LSTLFT EFTEST 5/23/2015Ardavan Asef-Vaziri5-1-30

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