Crashing project activities is a typical way to shorten their completion times to meet project due dates, and previous research on quality in time–cost tradeoff problems focused on maximizing the individual activity quality of projects. However, implementing project scheduling that takes into account the potential quality loss cost (PQLC) in time–cost tradeoff problems is a practical approach, since individual activity quality is defined by conformance to project contractor requirements. We propose a mixed integer linear programming model that considers the PQLC for excessive crashing activities. This model will help project planners develop practical project schedules.
Highlights
► Potential quality loss costs are significant factor in time–cost tradeoff problem. ► Model accounts for potential quality loss cost of project activities. ► Cost overruns of project activities can be avoided by using model. ► Model helps project planner develop practical project schedule.
Successful projects should be completed before project due dates and within budget; however, these limits are sometimes surpassed. There may therefore be significant variance between the assumptions made regarding a project and actual outcomes. Sudden unexpected changes in construction technology, techniques, materials, or human resources can create budgetary and scheduling pressures that in turn may increase the possibility of failure (Zeng et al., 2007). A survey exploring the completion of construction projects in Saudi Arabia showed that 76% of project contractors experienced delays of 10–30% of the projected duration (Assaf and Al-Hejji, 2006).
A typical technique used to mitigate scheduling pressure is to crash project activities. Crashing activities involves allocating more resources (such as materials, labor, and equipment) than planned in order to complete a project more quickly (Kessler and Chakrabarti, 1999).
In time–cost tradeoff problems, projects are not always completed as scheduled without reworking or modification. A project is a one-time task constrained by time, cost, and quality, and its success depends on how well these constraints are balanced (Atkinson, 1999). If any one of the constraints is overemphasized, burdens may fall on the other two. Hence, crashing project activities should be considered a significant factor in the time–cost tradeoff problem.
Some previous studies have treated quality as an important factor in tradeoff problems, claiming that overall project quality attained by project activities should be maximized within a given deadline and budget. These studies have also promoted using a continuous scale from zero to one to specify the quality of each activity (Babu and Suresh, 1996, Tareghian and Taheri, 2006 and Tareghian and Taheri, 2007).
However, a study evaluating the application of the time–cost–quality tradeoff model to linear programming for a cement factory construction project in Thailand revealed two facts: overall project quality cannot be sacrificed by crashing, and individual activity quality is primarily determined by subjective judgements, with the exception of a few measurable activities (Khang and Myint, 1999). In real situations, even if overall project quality meets project quality targets, if any single project activities do not meet the project contractor's requirements, rework or modification may be necessary and are associated with time delay and cost overrun. The possibility of rework or modifications must be considered when crashing project activities to develop practical and cost effective project schedules.
This paper proposes a mixed integer linear programming model and procedure that accounts for potential quality loss cost (PQLC) associated with rework or modifications that may occur due to excessive crashing activities. The rest of the paper is organized as follows. Section 2 summarizes previous research related to time–cost tradeoff problems considering project quality. Section 3 describes the mixed integer linear programming model used to compute direct project costs, which was previously considered to be equal to the nonconformance activity rate of the project activities. Section 4 validates this model with an example and discusses the PQLC estimation method of the example project. Section 5 provides the conclusions of the study.
In this paper, we propose a mixed integer linear programming model that accounts for both the nonconformance risks and the PQLC of the project activities. In a computational application of this model we found that the crashing activities were properly selected, and identified a need for special care regarding nonconformance risk activities. By identifying these nonconformance risk activities in the process of project scheduling, the project manager can take preventive actions that eliminate the need for rework or modification, which in turn facilitates the completion of the RTPSI project with minimum direct costs and before the due date. Although rework or modification of nonconformance risk activities cannot be completely avoided during project execution, project cost overruns can be avoided because the direct cost already includes the PQLC. This is similar to a worst-case design concept in terms of reliability.
In conclusion, we present a valid and practical model that can minimize PQLC influence on project cost due to excessive crashing activities. In an example project, we determined that quality is a significant factor in time–cost tradeoff problems when project activities are excessively crashed. When compared with the previous models in Table 1, the proposed model will help project planners develop practical project schedules, and is a meaningful approach in view of its possible utilization in real life projects. Preventive actions for nonconformance risk activities may incur additional cost. In future research, the proposed model may be extended to estimate the additional costs of preventative actions that are related to nonconformance risk activities and to use this information to formulate new approaches that incorporate quality concepts into project scheduling problems.
