Questions and Answers
What is the importance of determining activity durations in project scheduling?
The accuracy of the schedule is directly related to the accuracy of individual activity durations.
What are the two primary factors that determine the soundness of a detailed cost estimate?
Conceptual estimates are typically used by contractors prior to submitting bids for contracts.
False
In detailed cost estimates, the costs of materials, labor, equipment, subcontracted work, overhead, and profit are included. This thorough breakdown helps in proper ________ of the project.
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What type of costs are included under 'Equipment Costs' on a job site?
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What expenses are categorized under 'Temporary Protection and Safety Requirements'?
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A reasonable approximation of actual costs must be maintained for home-office overhead on all company projects.
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What is essential for accurately estimating project costs?
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What is the formula for calculating forming time?
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What resource should a scheduler explore for relevant information before using published reference materials?
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Rounding up to whole integer values when estimating durations adds conservatism to the duration estimate.
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Float is the leeway assigned to various activities in a network to manage __________.
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Match the following scheduling considerations with their descriptions:
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How might the duration be determined for the clearing and grubbing on the site of a small building project?
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What might be assumed when determining the duration for wallpaper installation on one accent wall in a reception area?
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What method is suggested if the scheduler has no experience with a particular work item?
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Reference guides for productivity estimates are highly accurate in predicting durations for all project scenarios.
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What is the production rate for the excavation activity (in hours per cubic meter)?
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Match the following activities with their duration estimation methods:
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What is the area to the left of the mean on a standard normal curve?
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If the area under a bell-shaped curve to the right of the mean is encountered, what happens to the schedule?
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According to Table 4.3, the area to the left of two standard deviations is ____ percent of the total area.
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A confidence level of 97 percent corresponds to being one standard deviation beyond the mean.
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Match the following data with the corresponding contractors:
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What estimate should be used if we are to have a 95 percent confidence that the estimated duration will not be exceeded?
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What duration (in working days) should be used for the entire scaffolding erection, if 10 workers are assigned to the task?
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If three workers are assigned to install tiles on a lobby floor, what duration (to the nearest whole day) should be used to complete the task?
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What duration should be used to ensure a 95 percent confidence that the duration of the activity will not exceed 10 days?
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What duration should be used to ensure a 95% confidence that the task of framing a building will not be exceeded?
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How many days should be allowed to complete the masonry facade task with a crew of six workers?
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Study Notes
Determining Activity Durations
- Activity durations are crucial for project scheduling, and their accuracy directly affects the schedule's value.
- There are several methods to obtain or derive activity durations, and some degree of uncertainty is always present.
Estimating
- Estimating is the process of determining the probable cost of a project, and it involves preparing construction estimates before a project is constructed.
- There are two types of estimates: conceptual estimates and detailed estimates.
Conceptual Estimates
- Conceptual estimates are rough approximations of the anticipated costs, prepared by architects, engineers, or consultants to help owners decide on a project.
- They are used to determine the approximate cost of a project before making a final decision to construct it.
Detailed Estimates
- Detailed estimates are prepared by contractors prior to submitting bids on competitively bid contracts or when entering lump-sum or fixed-price contracts.
- They include the costs of materials, labor, equipment, subcontracted work, overhead, and profit.
- A detailed estimate begins with a thorough analysis of the physical quantities that must be incorporated into the final project.
Conducting a Detailed Estimate
- The process involves a detailed quantity take-off, which is recorded on a standard sheet that includes information about the various items.
- Costs are captured separately for materials, equipment, labor, and subcontracted items.
- Separating cost items makes it easier to check and verify that all cost categories have been included.
Types of Estimates
- Construction estimates can be divided into two types: conceptual estimates and detailed estimates.
- Conceptual estimates are used to determine the approximate cost of a project before making a final decision to construct it.
- Detailed estimates are used to prepare a detailed cost estimate for a project.
Job Site Overhead
- Job site overhead includes all costs that cannot be attributed to a specific work activity but must be incurred to successfully complete the project.
- Examples of job site overhead items include the cost of maintaining a job pickup truck, the salary of the superintendent, and the cost of an electric generator.
Home-Office Overhead
- Home-office overhead includes all costs incurred at the home office that cannot be attributed easily to one project.
- Examples of home-office overhead items include salaries, equipment costs, building or office costs, supplies, operating expenses, and other expenses.
- The allocation of home-office overhead to a project begins with a separate estimate of the anticipated cost to maintain the home office.
Importance of Scheduling
- It is essential to prepare an accurate estimate of the project duration, as many of the costs of a project are a direct function of the amount of time required to complete the project.
- Preparing a schedule with sufficient detail is crucial for a reasonably accurate estimate of the project duration.### Constructing Walls and Slabs
- The sequence of wall construction activities: construct wall footing, place granular fill, finish grade, erect form wall, place concrete wall, place concrete slab, backfill, and cleanup.
- The above sequence can be identified by analyzing the work plan.
Determining Activity Durations
- Activity durations can be determined by analyzing the operation and quantities involved.
- Schedulers may make an educated guess about the duration of a task based on experience, but detailed analysis is necessary for accuracy.
- The duration of small tasks can be determined with confidence after a limited evaluation.
- For example, clearing and grubbing a one-acre site with no trees, debris, or vegetation may take 4 days to complete, including 2 days for equipment mobilization.
Estimating Durations
- The method of establishing duration depends on the size of the activity and required accuracy.
- For large activities, detailed analysis is necessary to ensure accuracy.
- For small activities, a rough estimate can be made based on past experiences.
- Allowances for mobilization, preparation, and cleanup should be included in the duration estimate.
Scheduling Issues
- Determining activity durations is important, but it's also important to consider the actual scheduling of activities.
- Float is the leeway assigned to various activities in a network, which can be manipulated by the project manager.
- An early-start schedule allows for flexibility, while a late-start schedule removes all flexibility.
- A compromise between the two schedules may be the ideal solution.
Factors Influencing Choice of Activity Schedule
- Scheduling should satisfy various constraints, such as physical, resource, environmental, management, safety, and contract constraints.
- Activity start times are established to reflect these constraints.
- The scheduler must balance the relative uncertainties inherent in a construction project to develop a realistic schedule.
Cash Flow and Materials Delivery
- The cash flow diagram can be used to predict cash flow requirements on the project.
- Materials delivery and storage should be considered in the schedule.
- Staggering deliveries can reduce job site congestion and simplify coordination efforts.
- The schedule should reflect significant concerns related to materials.
Other Considerations
- The project manager should anticipate potential problems and develop a plan to minimize their impact.
- Historical data on past projects can be a valuable resource for the scheduler.
- The scheduler should exercise judgment when determining activity durations and consider the need for rework and inspections.### Scheduling and Weather Considerations
- Weather considerations are important in scheduling as they can impact project timelines
- Management may anticipate the need for certain precautionary measures, including those guarding against weather effects (rain, snow, temperature extremes, high winds, and poor soil conditions)
- Schedule may be adjusted to:
- Winterize a portion of the project to stay on schedule
- Handle poor cash positions by scheduling expensive noncritical activities later
- Account for potential delays in material deliveries and labor agreements
Incorporating Weather Contingencies
- Weather-dependent activities can be scheduled earlier or later in the project timeline
- Increasing the duration of weather-dependent activities by a percentage based on the likelihood of delay
- Inserting specific weather delay activities into the network schedule
Uncertainty in Duration Estimates
- Activity durations are assumed to be fixed, but there is a fundamental flaw in this approach
- Duration estimates are not always accurate and may be affected by various factors (e.g., weather, material deliveries, labor agreements)
- Understanding the true meaning of duration estimates requires appreciation of probability and statistics
Probability and Statistics
- Table 4.3 provides information on the probabilities associated with certain estimates, defined in terms of standard deviation
- The area under the bell-shaped curve can be used to determine the probability of completing a task within a certain timeframe
- For example, with a duration estimate of one standard deviation beyond the mean, there is an 84.13% chance of completion within the allotted time
Determining Activity Durations
- The mean and standard deviation of past project data can be used to estimate activity durations
- Contractors may use different methods to estimate durations, but the essence of their logic is similar to the statistical approach
- Owners may view durations differently, expecting a higher level of confidence in the estimates
Case Study Examples
- Estimating the duration of setting flatwork (sidewalk) forming on a project based on past project data
- Determining the production rate for erecting scaffolding on a six-story building
- Calculating the duration of installing tiles in a large lobby area based on the production rate and number of workers
- Estimating the duration of constructing a masonry facade based on the production rate and labor cost
- Making allowances for adverse weather days in a project schedule
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Description
This quiz covers the importance of determining activity durations in project management, including planning and scheduling to estimate project costs.
