Reducing project duration at minimum cost: A time-cost tradeoff algorithm

The problem of reducing project duration efficiently arises frequently, routinely, and repetitively in government and industry. Siemens [1] has presented an inherently simple time-cost tradeoff algorithm (SAM—for Siemens Approximation Method) for determining which activities in a project network must be shortened to meet an externally imposed (scheduled) completion date (which occurs prior to the current expected completion date). In that paper the network activities of the example problem all have constant cost-slopes. Siemens mentions that the algorithm can be used where the activities have (convex) nonlinear cost-slopes—instead of just one cost-slope and one supply (time available for shortening) for each activity, there can be multiple cost-slope/supply pairs for each activity. This technique is illustrated in this paper. Also illustrated here is an improvement suggested by Goyal [2]. In step 12 of the original algorithm Siemens suggests a review of the solution obtained by the first eleven steps to eliminate any unnecessary shortening. Goyal's modification does this systematically during application of the algorithm by de-shortening (partially or totally) selected activities which were shortened in a prior iteration. He claims that, empirically at least, the technique always yields an optimal solution. Our experience verifies this claim (given the assumption of convex cost functions). The authors have modified the original algorithm so that the requirement for convex cost functions can now be relaxed. Unfortunately, this modification is made only at the expense of simplicity. To further complicate matters we found that Goyal's technique does not always yield an optimal solution when concave functions are involved and thus still another modification was required. These are discussed in detail below. Finally, we discuss the applicability of the algorithm to situations involving discrete time-cost functions.     

Choosing the Right Expert System Building Approach

The analytical hierarchy process (AHP) is used to systematically examine the relative appropriateness of contending expert system building approaches—algorithmic programming languages, symbolic programming languages, expert system shells, and knowledge engineering environments based upon characteristics of application domains and development environments. Two expert system projects, Back Expert, and R&D Expert, illustrate the value of this methodology. Domain and development environment characteristics identified in the AHP approach are then used to construct rule-based and Lotus 1-2-3 expert systems for selecting the right expert system building approach.     

Modeling Take-or-Pay Contract Decisions

Long-term purchase contracts for natural resources, such as natural gas, often contain take-or-pay provisions that penalize the buyer for not purchasing (taking) a minimum quantity of output over some period of time. In some contracts for a limited time interval, known as the make-up period, the penalty payments can be credited against future “takes” in excess of the take-or-pay level. Additionally, options to “buy down” or “buy out” existing contracts, or to initiate new contracts, may exist. The purchaser, faced with projected requirements over some planning horizon, must determine purchase levels from a selected set of take-or-pay contracts so as to minimize purchase, inventory holding, penalty costs, contract initiation, and buy-out or buy-down costs. This paper presents a mixed-integer programming model of take-or-pay decisions with and without make-up provisions.     

Permanent Disadvantage or Gradual Integration: Explaining the Immigrant–Native Earnings Gap in Sweden

Theoretical explanations suggest that wage differentials between immigrant and native workers are generated either by differences in the acquisition of human capital or by various forms of exclusion of immigrants from fair labour market rewards. We evaluate the labour quality and labour market discrimination hypotheses by using a large sample of Swedish employees in 1995. Our findings show that labour market integration is relatively unproblematic for immigrants from Western countries, whereas immigrants from other countries, especially from Africa, Asia and Latin America, face substantial obstacles to earnings progress when entering the Swedish labour market. For the latter group of countries, extensive controls for general and country‐specific human capital reduce the earnings differentials. However, the remaining gap is of a non‐trivial magnitude. Thus, the labour quality hypothesis accounts for a part of the observed native–immigrant wage gap, but the remaining differentials can be interpreted in terms of labour market discrimination.     

Relationship course theory: An interdisciplinary integrative proposition to address the complexification of interpersonal relationships

Researchers have been observing a complexification of interpersonal relationships in contemporary societies. However, current theoretical perspectives on relationships fall short of comprehensively grasping increasingly diverse and fluid relationship types (e.g., friends with benefits, polyamory, living-apart-together, coparenting, etc.) and patterns of change. In an attempt to meet the need for more integrative and interdisciplinary theorizing, this paper introduces a first outline of relationship course theory to better comprehend the complexity of relationships. In contrast to previous theoretical perspectives that conceptualize relationships as a single trajectory, I posit that the course of a relationship is composed of multiple intertwined trajectories, each of which stemming from differentiated domains such as sexuality, friendship, love, family, domesticity, and occupation. These relationship domains constitute a metatypology from which to examine the multiple sets of meanings and temporalities that intertwine throughout the course of a relationship. I propose that relationships are defined based on the combination of relationship domains, with different iterations and permutations of these domains producing various relationship configurations. Furthermore, the theory defines three levels of relationship courses: Sociocultural (i.e., stories that circulate about relationships in given sociocultural contexts), interpersonal (i.e., stories that individuals co-construct about the particular relationships they experience), and life courses (i.e., stories that individuals co-construct about themselves as relational beings). This proposition will hopefully stimulate the theoretical conversation on the complexity of relationships and foster dialogue between researchers from different theoretical and disciplinary affiliations.