Duration and cost variability of construction activities: an empirical study

The unique nature of construction projects can mean that construction activities often suffer from duration and cost variability. Because this variability is unplanned, it can present a problem when attempting to complete a project on time and on budget. Various factors causing this variability have been identified in the literature, but they predominantly refer to the nature and/or context of the whole project rather than specific activities. In this paper, the order of magnitude of and correlation between activity duration and cost variability is analyzed in 101 construction projects with over 5,000 activities. To do this, the first four moments (mean, standard deviation, skewness, and kurtosis) of actual versus planned duration and cost (log) ratios are analyzed by project, phase of execution, and activity type. Results suggest that, contrary to common wisdom, construction activities do not end late on average. Instead, the large variability in the activity duration is the major factor causing significant project delays and cost overruns. The values of average activity duration and cost variability gathered in this study will also serve as a reference for construction managers to improve future construction planning and project simulation studies with more realistic data

Setting tolerance limits for statistical project control using earned value management

Project control has been a research topic since decades that attracts both academics and practitioners. Project control systems indicate the direction of change in preliminary planning variables compared with actual performance. In case their current project performance deviates from the planned performance, a warning is indicated by the system in order to take corrective actions. Earned value management/earned schedule (EVM/ES) systems have played a central role in project control, and provide straightforward key performance metrics that measure the deviations between planned and actual performance in terms of time and cost. In this paper, a new statistical project control procedure sets tolerance limits to improve the discriminative power between progress situations that are either statistically likely or less likely to occur under the project baseline schedule. In this research, the tolerance limits are derived from subjective estimates for the activity durations of the project. Using the existing and commonly known EVM/ES metrics, the resulting project control charts will have an improved ability to trigger actions when variation in a project׳s progress exceeds certain predefined thresholds A computational experiment has been set up to test the ability of these statistical project control charts to discriminate between variations that are either acceptable or unacceptable in the duration of the individual activities. The computational experiments compare the use of statistical tolerance limits with traditional earned value management thresholds and validate their power to report warning signals when projects tend to deviate significantly from the baseline schedule

On the duration and cost variability of construction activities: an empirical study

The unique nature of construction projects can mean that construction activities often suffer from duration and cost variability. As this variability is unplanned it can present a problem when attempting to complete a project on time and on budget. Various factors causing this variability have been identified in the literature, but they predominantly refer to the nature and/or context of the whole project, rather than their specific activities. In this paper, the order of magnitude of and correlation between activity duration and cost variability is analyzed in 101 construction projects with over 5000 activities. To do this, the first four moments (mean, standard deviation, skewness and kurtosis) of actual versus planned duration and cost (log) ratios are analyzed by project, phase of execution and activity type. Results suggest that, contrary to common wisdom, construction activities do not end late on average. Instead, the large variability in the activity duration is the major factor causing significant project delays and cost overruns. The values of average activity duration and cost variability gathered in this study will also serve as a reference for construction managers to improve future construction planning and project simulation studies with more realistic data

Schedule network node time distributions and arrow criticalities

This research develops exact methods to calculate project duration distributions and to calculate Van Slyke\u27s (1963) criticality for arrows, the probability that an arrow is on a critical path, assuming nonnegative integer duration distributions. These calculations for project duration distributions correct estimates made by the Program Evaluation and Review Technique (PERT), and the Van Slyke criticality calculations extend the arrow criticality analysis by the Critical Path Method (CPM) into the probabilistic realm;Exact methods for calculating project duration distributions and Van Slyke\u27s criticality are demonstrated on series networks, parallel networks, parallel-series networks, and the Wheatstone network. The Van Slyke criticality equation for parallel networks is in a form that appears to improve upon one proposed by Dodin & Elmaghraby (1985). The present form is generalized to, in principle, include all networks;The exact methods are enhanced by developing a procedure to limit the number of calculations needed to analyze large networks. The procedure identifies paths through a large network, calculates the minimum and maximum path durations, and ranks the paths by duration. A smaller skeletal network is constructed from the arrows of the longest paths and is analyzed by exact methods. The procedure emphasizes accuracy for the longer project durations, of greatest concern to project managers and schedulers, while limiting the number of necessary calculations;The procedure for large networks is illustrated on the 40-arrow Kleindorfer (1971) network. Of the 51 Kleindorfer paths, the procedure selected 6 paths to construct a skeletal network. Analysis of the skeletal network yields a project duration distribution that is correct in its range and in the duration probabilities for the upper 5% of the distribution. Analysis results are compared with SLAM II and FORTRAN simulations. No arrow criticality appears to be seriously miscalculated. The project duration distribution is calculated to be bimodal, in keeping with the simulation;Conditions under which the just mentioned bimodality can occur are determined for parallel, normally-distributed paths. The large-network procedure warns when these oddly shaped distributions are possible

M-PERT: manual project-duration estimation technique for teaching scheduling basics

The Program Evaluation and Review Technique (PERT) has become a classic Project Management tool for estimating project duration when the activities have uncertain durations. However, despite its simplicity and widespread adoption, the original PERT, in neglecting the merge event bias, significantly underestimated the duration average and overestimated the duration variance of real-life projects. To avoid these and other shortcomings, many authors have worked over the last 60 years at producing interesting alternative PERT extensions. This paper proposes joining the most relevant of those to create a new reformulated PERT, named M-PERT. M-PERT is quite accurate when estimating real project duration, while also allowing for a number of interesting network modelling features the original PERT lacked: probabilistic alternative paths, activity self-loops, minima of activity sets and correlation between activities. However, unlike similar scheduling methods, M-PERT allows manual calculation through a recursive merging procedure that downsizes the network until the last standing activity represents the whole (or remaining) project duration. Hence, M-PERT constitutes an attractive tool for teaching scheduling basics to engineering students in a more intuitive way, with or without the assistance of computer-based simulations or software. One full case study will also be proposed and future research paths suggested

Analysis of Integrated Reliability for Heterogeneous Events

학위논문 (박사)-- 서울대학교 대학원 : 생태조경·지역시스템공학부(지역시스템공학전공), 2015. 8. 이정재.본 연구는 불확실성을 갖는 개별 확률사상들이 결합된 자연현상을 개별 요소간의 인접성과 관계없이 연결성의 지배를 받는 위상구조와, 인접요소들과의 물리적 거리의 지배를 받는 기하구조로 구분하고, 이들에 대해 확률재규격화를 이용한 시스템 신뢰성해석 방법을 제안하였다. 일반적으로 대부분의 공학문제는 결정론적 방법론을 통해 자연현상을 설명하는 지배방정식을 정의하고, 이를 구성하는 계수와 초기조건, 경계조건을 지배방정식에 대입하여 해석한다. 그러나 지배방정식에 필요한 여러 조건과 계수는 측정 혹은 추정에 의존해야 하므로 불확실성을 내포하게 된다. 불확실성을 고려하기 위하여 확률론적 해석에 기반한 신뢰성 해석기법들이 여러 분야에 도입되었으나, 단순 사상이 아닌 여러 사상이 결합되어 있는 문제에서는 이를 적용하기 어려웠다. 몬테카를로 시뮬레이션 등의 기법으로 계산하는 방법도 제안되었으나, 복합사상의 복잡성이 증가할수록 계산량도 기하급수적으로 증가하는 한계가 있었다. 따라서 본 연구에서는 복합사상을 위상구조 문제와 기하구조 문제로 단순화 하고 확률재규격화를 통해 신뢰성해석 하는 방법을 제시하였다. 위상구조의 복합확률사상은 계층구조로 변환한 후, 같은 상위 사상을 공유하는 하위 확률사상들을 확률재규격화를 이용해 결합함으로써 상위 사상의 확률사상을 산정할 수 있고, 이를 반복하여 시스템 전체의 신뢰성을 해석할 수 있다. 본 연구에서는 이에 대한 적용성을 고찰하기 위하여 농업용저수지의 재해에 대한 위험도를 홍수, 지진으로 인한 제체붕괴, 수위급강하로 인한 제체붕괴의 조건에 대하여 분석하였다. 기하구조의 복합확률 사상은 미분방정식으로 정의되는 연속체 문제를 신뢰성 해석하는 과정이다. 미분방정식을 수치해석하기 위해 유한차분법, 유한요소법 등의 방법으로 이산화하면, 연속체 문제를 이산화된 인접요소간의 정보교환과정으로 단순화 할 수 있다. 이들 개별 요소의 값이 확정값이 아닌 불확실성을 내포한 값인 경우, 이 개별 요소의 확률사상을 확률재규격화를 통해 계산할 수 있다. 이 방법론에 대한 검증을 위하여 경계조건과 초기조건이 확률분포로 존재하는 열확산문제에 적용하였으며, 활용예로 배수갑문 콘크리트 구조물의 염화이온 확산문제에 적용하고 시간에 따른 콘크리트 깊이별 신뢰도를 산정하였다. 본 논문에서 제안하는 방법은 시스템 전체의 신뢰도 평가와 시스템을 구성하는 개별 요소들의 신뢰도 평가를 동시에 수행할 수 있다는 이점이 있다. 이를 이용해 다양한 지역의 재난에 대한 위험도를 확률적으로 평가하거나, 구조물의 취약점을 찾아내는 등으로 활용될 수 있을 것이다.가중확률재규격화를 이용한 복합사상의 신뢰성해석 1 국 문 초 록 1 1 서 론 9 1.1 연구배경 9 1.2 연구내용 및 범위 12 1.3 논문구성 14 2 연구사 16 2.1 확률론 16 2.2 위험도 18 2.3 확률재규격화 19 2.4 복합사상과 네트워크 21 2.4.1 위상구조의 복합확률사상 22 2.4.2 기하구조의 복합확률사상 23 3 복합사상과 확률 재규격화 25 3.1 확률복합사상과 가중확률 재규격화 25 3.2 가중확률재규격화의 적용 28 3.2.1 위상구조를 갖는 복합사상의 적용 28 3.2.2 기하구조를 갖는 복합사상의 적용 29 4 위상구조를 갖는 복합확률사상의 해석 30 4.1 확률복합사상과 위험도 30 4.1.1 위험도의 계층구조화 30 4.1.2 가중확률재규격화 31 4.1.3 비모수적 추정 방법을 통한 확률분포의 추정 31 4.2 재해 가중확률재규격화 34 4.2.1 복합사상을 이용한 재해 분석과 연구배경 34 4.2.2 재해와 시스템 37 4.2.3 발생확률 37 4.2.4 가중치 38 4.2.5 위험도 지수 (β) 38 4.3 위상구조를 갖는 사상의 적용과 고찰 39 4.3.1 저수지 연관지역 위험도 평가의 개념과 범위 40 4.3.2 재해별 발생확률 산정 46 4.3.3 재해별 가중치 산정 50 4.3.4 적용 결과 54 4.4 소결 62 5 기하구조를 갖는 복합확률사상의 해석 64 5.1 기하구조 문제와 확률수치해석 64 5.2 확산 방정식과 차분 65 5.2.1 확산방정식과 유한차분해석 65 5.2.2 양함수 유한차분해석 선정 이유 67 5.3 확률재규격화를 이용한 확률유한차분해석 68 5.4 기하구조 문제의 적용과 고찰 69 5.4.1 재규격화를 이용한 확률유한차분법의 검증 69 5.4.2 배수갑문 염해해석 75 5.5 소결 88 6 종합결론 90 7 참고자료 92 Appendix 99 Abstract 104Docto