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Chapter 9 Network Analysis and Duration Estimating

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Objectives Laying the groundwork for commitment. Duration estimating. CPM versus PERT. Calculating Project Duration. Optimizing the Plan.

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Commitment Estimated duration: the estimated time needed to finish the project. A good project manager is constantly asking questions and getting input from his or her team members. Be sure to ask people who will actually do the work for time related questions. For example you would ask an electrician how long it takes to wire a 1,000 square foot space for 30 cubicles, not the person who orders cable. You should also look for the team member with the most experience in any given area. Those who have performed the task many times over know realistically how long an activity will take, and will give you the most accurate estimates. If you are in Florida, planning a job in Michigan, you should consult with the people in Michigan (frozen ground in winter).

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Duration Estimating Effort: the actual time spent on an activity. Duration: the elapsed time from the start of an activity until it is finished. Duration estimating: 1. Experience: The most commonly used estimating technique is to ask someone with experience. 2. Historical data 3. Research: Vendors are an excellent source of information. 4. Breakdown/roll-up: A complex task can be broken down into familiar steps as an aid to estimating the duration. Common problems: A 20% chance of rain means that you change the planned duration from 5 to 6 days.

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CPM versus PERT PERT (Program Evaluation and Review Technique): requires that you develop three duration estimates for each activity - the most optimistic, the most pessimistic, and the most likely. A probability is attached to each estimate, and the final duration is devised from the formula = DoPo + DpPp + DePe where Do is the most optimistic duration, Po is the probability of Do etc. CPM (Critical Path Method): is most often used in commercial projects. We use only the most likely duration. This tends to give results that are just as valid as PERT.

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Calculating Project Duration The critical path: The path with the longest cumulative duration will determine the length of a project. Forward pass calculation: the approach to determine the duration of the whole project. EPF = EPS + DUR where EPF is the Earliest Possible Finish time of an activity, EPS is the Earliest Possible Start time for an activity, and DUR is the Duration of an activity (fig 9-3, page 189). Backward pass calculation: allows the identification of slack time for each activity. A typical duration for a 3,000 square foot house is about 6 months. A typical duration for a manufacturing plant is about 15 months.

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Optimizing the Plan The Need to Meet External Deadlines: Would bringing more talent speed it up? Would round the clock scheduling shorten project duration? The First Line of Defense-Relieve the Critical Path: See if any activities can be taken off the critical path. Can some activities be started before their predecessor is completely finished? The Second Line of Defense-Sharpen Your Pencil: Push for tighter estimates. Usually when you ask team members to account for each detail of the work, the duration estimates will go down. The Third Line of Defense-Crashing Activities: Spending more money in order to shrink the duration of some activities to bring the whole project within deadline.

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Summary Estimated duration: the estimated time needed to finish the project. Those who have performed the task many times over know realistically how long an activity will take, and will give you the most accurate estimates. Duration: the elapsed time from the start of an activity until it is finished. Common problems: A 20% chance of rain means that you change the planned duration from 5 to 6 days. CPM (Critical Path Method): is most often used in commercial projects. We use only the most likely duration. This tends to give results that are just as valid as PERT. The critical path: The path with the longest cumulative duration will determine the length of a project. Forward pass calculation: the approach to determine the duration of the whole project. The Need to Meet External Deadlines: Would bringing more talent speed it up? The First Line of Defense-Relieve the Critical Path The Second Line of Defense-Sharpen Your Pencil The Third Line of Defense-Crashing Activities

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Home Work 1. Define the term “duration” of an activity? 2. Explain PERT (program evaluation and review technique). 3. Explain CPM (critical path method). 4. Explain critical path.

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