Network Theory - Project Scheduling PDF

Network design, Project Scheduling  INTRODUCTION Schedule converts action plan into operating time table Basis for monitoring and controlling project Scheduling more important in projects than in production, because unique...

Network design, Project Scheduling  INTRODUCTION Schedule converts action plan into operating time table Basis for monitoring and controlling project Scheduling more important in projects than in production, because unique nature Sometimes customer specified/approved requirement-e.g: JKR projects Based on Work Breakdown Structure (WBS) Chapter 8 Scheduling, PERT, Critical Path Analysis  NETWORK TECHNIQUES PERT CPM - Program Evaluation and Critical Path Method Review Technique Developed by El - developed by the US Dupont for Chemical Navy with Booz Plant Shutdown Hamilton Lockheed Project- about - on the Polaris same time as PERT Missile/Submarine program 1958 Both use same calculations, almost similar Main difference is probabilistic and deterministic in time estimation Chapter 8 Gantt Scheduling, Chart also used PERT, in Path Critical scheduling Analysis  NETWORK Graphical portrayal of activities and event Shows dependency relationships between tasks/activities in a project Clearly shows tasks that must precede (precedence) or follow (succeeding) other tasks in a logical manner Clear representation of plan – a powerful tool for planning and controlling project Chapter 8 Scheduling, PERT, Critical Path Analysis Example of Simple Network – Survey Chapter 8 Scheduling, PERT, Critical Path Analysis 5 Example of Network – More Complex Chapter 8 Scheduling, PERT, Critical Path Analysis 6  DEFINITION OF TERMS IN A NETWORK Activity : any portions of project (tasks) which required by project, uses up resource and consumes time – may involve labor, paper work, contractual negotiations, machinery operations Activity on Arrow (AOA) showed as arrow, AON – Activity on Node Event : beginning or ending points of one or more activities, instantaneous point in time, also called ‘nodes’ Network : Combination of all project activities and the events SUCCESSOR PRECEEDING ACTIVITY EVENT Chapter 8 Scheduling, PERT, Critical Path Analysis 7 Emphasis on Logic in Network Construction Construction of network should be based on logical or technical dependencies among activities Example - before activity ‘Approve Drawing’ can be started the activity ‘Prepare Drawing’ must be completed Common error – build network on the basis of time logic (a feeling for proper sequence ) see example below WRONG !!! CORRECT  Chapter 8 Scheduling, PERT, Critical Path Analysis 8  Example 1- A simple network Consider the list of four activities for making a simple product: Activity Description Immediate predecessors A Buy Plastic Body - B Design Component - C Make Component B D Assemble product A,C Immediate predecessors for a particular activity are the activities that, when completed, enable the start of the activity in question. Chapter 8 Scheduling, PERT, Critical Path Analysis  Sequence of activities Can start work on activities A and B anytime, since neither of these activities depends upon the completion of prior activities. Activity C cannot be started until activity B has been completed Activity D cannot be started until both activities A and C have been completed. The graphical representation (next slide) is referred to as the PERT/CPM network Chapter 8 Scheduling, PERT, Critical Path Analysis  Network of Four Activities Arcs indicate project activities A D 1 3 4 B C 2 Nodes correspond to the beginning and ending of activities Chapter 8 Scheduling, PERT, Critical Path Analysis 11  Example 2 Develop the network for a project with following activities and immediate predecessors: Activity Immediate predecessors A - B - C B D A, C E C F C Try to do for the G first five (A,B,C,D,E) activities D,E,F Chapter 8 Scheduling, PERT, Critical Path Analysis  Network of first five activities A D 1 3 4 E B C 5 2 We need to introduce a dummy activity Chapter 8 Scheduling, PERT, Critical Path Analysis 13  Network of Seven Activities 1 A 3 D 4 G 7 dummy E B C 5 F 2 6 Note how the network correctly identifies D, E, and F as the immediate predecessors for activity G. Dummy activities is used to identify precedence relationships correctly and to eliminate possible confusion of two or more activities having the same starting and ending nodes Dummy activities have no resources (time, labor, machinery, etc) – purpose is to PRESERVE LOGIC of the network Chapter 8 Scheduling, PERT, Critical Path Analysis  EXAMPLES OF THE USE OF DUMMYACTIVITY Network concurrent activities a a 2 1 2 1 Dummy b 3 b WRONG!!! RIGHT  Activity c not WRONG ! required for e a a e d 1 b 1 b e d c 2 c WRONG RIGHT !!!  RIGHT  Chapter 8 Scheduling, PERT, Critical Path Analysis 15  WRONG!!! RIGHT!!! a d a d 1 1 b e b 2 2 4 e c f c f 3 3 a precedes d. a and b precede e, b and c precede f (a does not precede f) Chapter 8 Scheduling, PERT, Critical Path Analysis 16  Scheduling with activity time Activity Immediate Completion predecessors Time (week) A - 5 B - 6 C A 4 D A 3 E A 1 F E 4 G D,F 14 H B,C 12 I G,H 2 Total …… 51 This information indicates that the total time required to complete activities is 51 weeks. However, we can see from the network that several of the activities can be conducted simultaneously (A and Chapter 8 B, for example). Scheduling, PERT, Critical Path Analysis  Earliest start & earliest finish time We are interested in the longest path through the network, i.e., the critical path. Starting at the network’s origin (node 1) and using a starting time of 0, we compute an earliest start (ES) and earliest finish (EF) time for each activity in the network. The expression EF = ES + t can be used to find the earliest finish time for a given activity. For example, for activity A, ES = 0 and t = 5; thus the earliest finish time for activity A is EF = 0 + 5 = 5 Chapter 8 Scheduling, PERT, Critical Path Analysis  Arc with ES & EF time EF = earliest finish time ES = earliest start time Activity 2 [ 0,5] A 5 1 t = expected activity time Chapter 8 Scheduling, PERT, Critical Path Analysis  Network with ES & EF time D[5,8] 2 5 3 E[ 1 0] 6 , G[1 4 5,6 F[ 1 ] 5 ,5] 4 0, 2 7 1 0 A[ 4 26] 4 C[5,9] 2 4, ] I[ 4 2 1 6 B[0 ,6] [ 9,21] 6 H 12 3 Earliest start time rule: The earliest start time for an activity leaving a particular node is equal to the largest of the earliest finish times for all activities entering the node. Chapter 8 Scheduling, PERT, Critical Path Analysis 20  Activity, duration, ES, EF, LS, LF EF = earliest finish time ES = earliest start time Activity 3 [ 5,9 ] C [ 8 ,12] 4 2 LF = latest finish LS = latest start time time Chapter 8 Scheduling, PERT, Critical Path Analysis  Latest start & latest finish time To find the critical path we need a backward pass calculation. Starting at the completion point (node 7) and using a latest finish time (LF) of 26 for activity I, we trace back through the network computing a latest start (LS) and latest finish time for each activity The expression LS = LF – t can be used to calculate latest start time for each activity. For example, for activity I, LF = 26 and t = 2, thus the latest start time for activity I is LS = 26 – 2 = 24 Chapter 8 Scheduling, PERT, Critical Path Analysis  Network with LS & LF time D[5,8] 5 2 3[7,10] 0] G[1 10,24 E[ 1 6, 10] 14[ 1[5 5,6] [ F 6, 0, 2 ] 5[ 0,5] ,6] 4[ 7 5] 4] A[ 26] 0, 4 I[2 4, C[5,9] 26] 4[8,12] 2 4 , 2[ 1 6 B[0 6[6 ,6] [ 9, 21] , 12 H [ 12,24] ] 12 3 Latest finish time rule: The latest finish time for an activity entering a particular node is equal to the smallest of the latest start times for all activities leaving the node. Chapter 8 Scheduling, PERT, Critical Path Analysis 23  Slack or Free Time or Float Slack is the length of time an activity can be delayed without affecting the completion date for the entire project. For example, slack for C = 3 weeks, i.e Activity C can be delayed up to 3 weeks (start anywhere between weeks 5 and 8). ,9] 3 5 C[ 2 [ 8,12] 4 ES LS EF EF 5 8 9 12 LF-EF = 12 –9 =3 LS-ES = 8 – 5 = 3 LF-ES-t = 12-5-4 = 3 Chapter 8 Scheduling, PERT, Critical Path Analysis Activity schedule for our example Activity Earliest Latest Earliest Latest Slack Critical start (ES) start (LS) finish finish (LS-ES) path (EF) (LF) A 0 0 5 5 0 Yes B 0 6 6 12 6 C 5 8 9 12 3 D 5 7 8 10 2 E 5 5 6 6 0 Yes F 6 6 10 10 0 Yes G 10 10 24 24 0 Yes H 9 12 21 24 3 I 24 24 26 26 0 Yes Chapter 8 Scheduling, PERT, Critical Path Analysis 25  IMPORTANT QUESTIONS What is the total time to complete the project? 26 weeks if the individual activities are completed on schedule. What are the scheduled start and completion times for each activity? ES, EF, LS, LF are given for each activity. What activities are critical and must be completed as scheduled in order to keep the project on time? Critical path activities: A, E, F, G, and I. How long can non-critical activities be delayed before they cause a delay in the project’s completion time Slack time available for all activities are given. Chapter 8 Scheduling, PERT, Critical Path Analysis Importance of Float (Slack) and Critical Path 1. Slack or Float shows how much allowance each activity has, i.e how long it can be delayed without affecting completion date of project 2. Critical path is a sequence of activities from start to finish with zero slack. Critical activities are activities on the critical path. 3. Critical path identifies the minimum time to complete project 4. If any activity on the critical path is shortened or extended, project time will be shortened or extended accordingly Chapter 8 Scheduling, PERT, Critical Path Analysis  Importance of Float (Slack) and Critical Path (cont) 5. So, a lot of effort should be put in trying to control activities along this path, so that project can meet due date. If any activity is lengthened, be aware that project will not meet deadline and some action needs to be taken. 6. If can spend resources to speed up some activity, do so only for critical activities. 7. Don’t waste resources on non-critical activity, it will not shorten the project time. 8. If resources can be saved by lengthening some activities, do so for non- critical activities, up to limit of float. 9. Total Float belongs to the path Chapter 8 Scheduling, PERT, Critical Path Analysis  PERT For Dealing With Uncertainty So far, times can be estimated with relative certainty, confidence For many situations this is not possible, e.g Research, development, new products and projects etc. Use 3 time estimates m= most likely time estimate, mode. a = optimistic time estimate, b = pessimistic time estimate, and Expected Value (TE) = (a + 4m + b) /6 Variance (V) = ( ( b – a) / 6 ) 2 Std Deviation (δ) = SQRT (V) Chapter 8 Scheduling, PERT, Critical Path Analysis  Precedences And Project Activity Times Immediate Optimistic Most Likely Pessimistic EXP Var S.Dev Activity Predecessor Time Time Time TE V  a - 10 22 22 20 4 2 b - 20 20 20 20 0 0 c - 4 10 16 10 4 2 d a 2 14 32 15 25 5 e b,c 8 8 20 10 4 2 f b,c 8 14 20 14 4 2 g b,c 4 4 4 4 0 0 h c 2 12 16 11 5.4 2.32 I g,h 6 16 38 18 28.4 5.33 j d,e 2 8 14 8 4 2 Chapter 8 Scheduling, PERT, Critical Path Analysis 30  The complete network d 6 2 (15,25) j a (8,4) (20,4) e (10,4) 1 f 7 3 b (14,4) (20,0) g c (4,0) (10,4) i (18,28.4) h 5 4 (11,5.4) Chapter 8 Scheduling, PERT, Critical Path Analysis 31  Figure 8-13 The complete Network EF=20 35 d 6 2 a (15,25) j (20,4) (8,4) b e 20 43 (20,0) (10,4) f CRIT. TIME = 43 1 3 7 (14,4) g c (4,0) (10,4) i (18,28.4) h 5 4 (11,5.4) 10 24 Chapter 8 Scheduling, PERT, Critical Path Analysis 32  Critical Path Analysis (PERT) Activity LS ES Slacks Critical ? a 0 0 0 Yes b 1 0 1 c 4 0 4 d 20 20 0 Yes e 25 20 5 f 29 20 9 g 21 20 1 h 14 10 4 i 25 24 1 j 35 35 0 Yes Chapter 8 Scheduling, PERT, Critical Path Analysis 33 Assume, PM promised to complete the project in the fifty days. What are the chances of meeting that deadline? Calculate Z, where Z = (D-S) / V Example, D = 50; S(Scheduled date) = 20+15+8 =43; V = (4+25+4) =33 Z = (50 – 43) / 5.745 = 1.22 standard deviations. The probability value of Z = 1.22, is 0.888 1.22 Chapter 8 Scheduling, PERT, Critical Path Analysis 34  What deadline are you 95% sure of meeting Z value associated with 0.95 is 1.645 D = S + 5.745 (1.645) = 43 + 9.45 = 52.45 days Thus, there is a 95 percent chance of finishing the project by 52.45 days. Chapter 8 Scheduling, PERT, Critical Path Analysis 35  Comparison Between CPM and PERT CPM PERT 1 Uses network, calculate float or slack, identify critical path and Same as CPM activities, guides to monitor and controlling project 2 Uses one value of activity time Requires 3 estimates of activity time Calculates mean and variance of time 3 Used where times can be Used where times cannot be estimated with confidence, estimated with confidence. familiar activities Unfamiliar or new activities 4 Minimizing cost is more important Meeting time target or estimating percent completion is more important 5 Example: construction projects, Example: Involving new activities building one off machines, ships, or products, research and etc development etc Chapter 8 Scheduling, PERT, Critical Path Analysis 36  BENEFITS OFCPM / PERT NETWORK Consistent framework for planning, scheduling, monitoring, and controlling project. Shows interdependence of all tasks, work packages, and work units. Helps proper communications between departments and functions. Determines expected project completion date. Identifies so-called critical activities, which can delay the project completion time. Chapter 8 Scheduling, PERT, Critical Path Analysis  BENEFITS OFCPM / PERT NETWORK (cont.) Identified activities with slacks that can be delayed for specified periods without penalty, or from which resources may be temporarily borrowed Determines the dates on which tasks may be started or must be started if the project is to stay in schedule. Shows which tasks must be coordinated to avoid resource or timing conflicts. Shows which tasks may run in parallel to meet project completion date Chapter 8 Scheduling, PERT, Critical Path Analysis  Gantt Charts Since 1917; Useful for showing work vs time in form of bar charts e.g. Can draw directly or from CPM/PERT network Chapter 8 Scheduling, PERT, Critical Path Analysis  Modified PERT/CPM diagram from network a d 1 2 6 7 Legend e [ Scheduled 3 Start ] Scheduled f Finish 3 - Actual b Progress 1 3 5 Ä Unavailable L Current Date à Milestone Scheduled c dummy ¨ Milestone 1 4 Achieved h 4 0 5 10 15 20 25 30 35 40 45 Days Chapter 8 Scheduling, PERT, Critical Path Analysis 40  GANTT CHART Chapter 8 Scheduling, PERT, Critical Path Analysis Chapter 8 Scheduling, PERT, Critical Path Analysis Chapter 8 Scheduling, PERT, Critical Path Analysis  Gantt Charts and CPM/PERT Networks Gantt Charts: Even though a lot of info, easy to read and , understand to monitor and follow progress. Not very good for logical constraints Should be used to COMPLEMENT networks, not replace Chapter 8 Scheduling, PERT, Critical Path Analysis  RESOURCE ANALYSIS AND SCHEDULING  Ability to carry out projects depend on the availability of resources  Analyze resource implication -How requirements can be met and changes needed  Use resources efficiently  Use network to give information about time, resources Chapter 8 Scheduling, PERT, Critical Path Analysis 45 and cost Activities D, E, F, G and H require fitters. Construct a bar chart with activities at their EST indicating person required and total float. D 22222222 E 222222 Activity F 22 G 2222 H 4 4 44 444 44444 0 5 10 15 20 Time Add up across all activities to get the total number of men required. Chapter 8 Scheduling, PERT, Critical Path Analysis 46  Total number of man required Convert the bar chart to a histogram 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 5 10 15 20 Time Resource analysis before scheduling Shows: i) Variation from week to week (fitters) ii) Maximum number of person required (12) during week 5-6 Chapter 8 Scheduling, PERT, Critical Path Analysis 47 Examine resource implication. Example If only 8 fitters are available at any period during the projec New bar chart: D 22222222 E 222222 Activity F 22 G 2222 H 4 4 44 444 44444 0 5 10 15 20 Time Chapter 8 Scheduling, PERT, Critical Path Analysis 48 Additional Restriction – no fitters available until the end of week 5. Revised Schedule: D 22222222 E 222222 Activity F 22 G 2222 H 4 4 44 444 44444 0 5 10 15 20 Time Chapter 8 Scheduling, PERT, Critical Path Analysis 49  Resource constraints relates to: 1. Variations in resource requirements 2. Resource availability Smaller variations: 3. Easier control of the job 4. Better utilization of resources Big variations: 5. Frequent moving of manpower 6. Require close control 7. Affect efficiency Chapter 8 Scheduling, PERT, Critical Path Analysis 50 Total number of man required 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Time Histogram showing large resource variations