L. Valadares Tavares

On the optimal management of project risk

The uncertainty of project networks has been mainly considered as the randomness of duration of the activities. However, another major problem for project managers is the uncertainty due to the randomness of the amount of resources required by each activity which can be expressed by the randomness of its cost. Such randomness can seriously affect the discounted cost of the project and it may be strongly correlated with the duration of the activity.In this paper, a model considering the randomness of both the cost and the duration of each activity is introduced and the problem of project scheduling is studied in terms of the projects discounted cost and of the risk of not meeting its completion time. The adoption of the earliest (latest) starting time for each activity decreases (increases) the risk of delays but increases (decreases) the discounted cost of the project. Therefore, an optimal compromise has to be achieved. This problem of optimization is studied in terms of the probability of the duration and of the discounted cost of the project falling outside the acceptable domain (Risk function) using the concept of float factor as major decision variable. This last concept is proposed to help the manager to synthetize the large number of the decision variables representing each schedule for the studied project. Numerical results are also presented for a specific project network.

The risk of delay of a project in terms of the morphology of its network

Abstract The risk of the duration of a project exceeding a certain limit has been studied since the early papers proposing the PERT method but no results are available relating the statistical distribution of the total duration with the morphological features of the network. In this paper, a set of indicators is proposed to describe the morphology of the project network and an experimental research is carried out to relate the distribution of the total duration with such indicators. A random generator of project networks is used as well as a model to produce graphical representations of the network. Major results are presented showing how sensitive are the parameters of the distribution of the total duration to the morphology of the network, allowing the project manager to estimate the risk of delay in terms of the proposed indicators. Thus, these results can be used as a decision aid to select alternative designs for the project network taking into account the correspondent risk of delay.

Partial duration series method revisited

Abstract Partial duration series have been used to estimate the relationship of flood magnitude to return period and this approach has been compared to the annual maxima method by previous authors who obtained distinct or even conflicting conclusions. The relative efficiency of these two methods is evaluated considering separately independent and autocorrelated exceedances. General recommendations concerning the truncation level are presented.

Optimal resource profiles for program scheduling

Abstract A program is defined as a set of interconnected projects and its scheduling is studied in this paper with the purpose of maximizing its total net present value. The expenditure required by each project is modelled by a continuous function of time and a term to penalize the time variation of such expenditure is also included in the objective function. The proposed method is based on Dynamic Programming using optimality conditions derived by the Calculus of Variations. The application of the obtained results is exemplified for the scheduling of a renewal program for the railways network in Oporto (Portugal).

A multi-stage non-deterministic model for project scheduling under resources constraints

Abstract Nowadays the general purpose of project scheduling is the development of decision-aids to help managers improving the planning, the programming and the control of their projects. This target is hard to be achieved because projects have complex descriptions and because such decisions include multiple choices about the allocation of resources and the project calendar, which have to be improved in terms of several criteria such as total duration, net present value, risk of failing to meet the planned durations for important fractions of the project, etc. A review of the major approaches adopted to describe projects in terms of time and resources is presented in this paper showing that, unfortunately, too large deterministic models are usually obtained for projects with a realistic size. A new formulation of the general problem of project scheduling under resources constraints is proposed in this paper considering the project as a sequence of stages. According to this approach, the project can be studied as a sequence of decisions on resources allocation where each time interval between successive decisions (stage duration) is explicitly formulated as a decision variable. This formulation is very powerful and flexible as it avoids large amounts of binary variables and it allows progressive levels of detail concerning the discretization of the project schedule. Also, non-deterministic attributes can be introduced and the optimization problem can be easily solved for most common objective functions. Analytical solutions can also be deduced using the proposed model if specific hypotheses about the restrictions on durations and resources are assumed.

On the development of educational policies

Abstract The development of educational policies is studied in this paper using the system approach. The most promising areas for application of OR in policy design and evaluation are identified and discussed in terms of illustrative data taken from several countries, namely member states of the European Community.

A non-Gaussian Markovian model to simulate hydrologic processes

A Markovian process {Xt; t = 0,1,2…} following a negative exponential distribution and with an exponentially decaying autocorrelation function is proposed and described in this paper. This model can be useful to simulate autocorrelated and skewed hydrologic variables. To show this a successful application to model direct runoff peaks is presented. Some interesting theoretical properties of this process concerning up-crossing intervals and the distributions of extremes are also analysed herein.

The exact distribution of extremes of a non-gaussian process

The exact distribution of extremes of a non-gaussian stationary discrete process is obtained and their crossing intervals are studied in terms of the autocorrelation coefficients for any level of crossing. This process is an important model for some physical magnitudes.

A review of major paradigms and models for the design of civil engineering systems

Abstract In this paper, the author presents the five classical paradigms of the process of design in civil engineering and identifies a new emerging paradigm: the interactive multi-attribute learning paradigm. This paradigm is studied in terms of actors, structures and OR instruments which can help to fulfil its application to modern design of civil engineering systems.

A stochastic model to control project duration and expenditure

Abstract Many models of Project Management and Scheduling have recognized the non-deterministic nature of the duration of activities but, unfortunately, have adopted deterministic descriptions for resources and expenditures. In this paper, a stochastic model is proposed considering the stochastic nature of durations and expenditures as well as their statistical interdependence. Advanced analytical results are deduced to assess the financial risk of a project as well as to plan and to control its financial profiles.