http://repub.eur.nl/res/aut/3039/ List of Publications en http://repub.eur.nl/static-eur/img/logo.png http://repub.eur.nl http://repub.eur.nl/res/pub/21933/ 2010-12-22T00:00:00Z In a recent paper Gutiérrez et al. (2008) show that the lot-sizing problem with inventory bounds can be solved in O(T log T) time. In this note we show that their algorithm does not lead to an optimal solution in general. http://repub.eur.nl/res/pub/21936/ 2010-12-22T00:00:00Z Within the framework of reverse logistics, the classic economic lot-sizing problem has been extended with a remanufacturing option. In this extended problem, known quantities of used products are returned from customers in each period. These returned products can be remanufactured, so that they are as good as new. Customer demand can then be fulfilled both from newly produced and remanufactured items. In each period, we can choose to set up a process to remanufacture returned products or produce new items. These processes can have separate or joint set-up costs. In this paper, we show that both variants are NP-hard. Furthermore, we propose and compare several alternative MIP formulations of both problems. Because ‘natural’ lot-sizing formulations provide weak lower bounds, we propose tighter formulations, namely shortest path formulations, a partial shortest path formulation and an adaptation of the (l, S, WW)-inequalities for the classic problem with Wagner-Whitin costs. We test their efficiency on a large number of test data sets and find that, for both problem variants, a (partial) shortest path type formulation performs better than the natural formulation, in terms of both the LP relaxation and MIP computation times. Moreover, this improvement can be substantial. http://repub.eur.nl/res/pub/12548/ 2008-04-29T00:00:00Z In this paper we derive a new structural property for an optimal solution of the economic lot-sizing problem with time-invariant cost parameters. We show that the total holding cost in an order interval of an optimal solution is bounded from above by a quantity proportional to the setup cost and the logarithm of the number of periods in the interval. Since we can also show that this bound is tight, this is in contrast to the optimality property of the economic order quantity (EOQ) model, where setup cost and holding cost are perfectly balanced. Furthermore, we show that this property can be used for the design of a new heuristic and that the result may be useful in worst case analysis. http://repub.eur.nl/res/pub/18017/ 2003-01-01T00:00:00Z In this paper we consider the capacitated lot-sizing problem (CLSP) with linear costs. It is known that this problem is NP-hard, but there exist special cases that can be solved in polynomial time. The purpose of this paper is twofold. First, we derive a backward algorithm based on the forward algorithm by Chen et al. (1994), which solves the general CLSP. We give a problem instances that requires exponential running time using the backward algo- rithm. Second, we provide a new O(T2) algorithm for the CLSP with non-increasing setup cost, general holding cost, non-increasing production cost and non-decreasing capacities over time. This is done by adapting the backward algorithm. Numerical tests show the better performance of our algorithm compared to the algorithm proposed by Chung and Lin (1988). http://repub.eur.nl/res/pub/8811/ 1998-01-01T00:00:00Z BACKGROUND: Little is known about the validity of estimates of morbidity experienced at home. METHODS: In the Dutch National Survey of Morbidity and Interventions in General Practice mothers of 1630 children answered a health interview and kept a health diary for 3 weeks (only the first 2 weeks were used). Children's symptoms were recorded during the interview using a check list and monitored in the health diary through open-ended questions. RESULTS: In the interview parents reported symptoms for 65% of their children and in the diary for 54% of children. Ear problems, colds, fever and weakness and anxiety were reported more often in the interview. Mother's mental health was assessed by the General Health Questionnaire; those scoring >4 were assessed as having impaired mental health and these parents reported symptoms for more children in the interview (81%) than in the diary (65%). For similar reference periods, the least educated mothers reported fewer children with symptoms in the diary (45%) than in the interview (66%). More highly educated mothers reported similarly in the diary (67%) and the interview (70%). CONCLUSION: Both data collection methods yield different estimates of community morbidity. Explanations such as telescoping, the seriousness of the symptoms, the amount of psychological distress of the respondent, forgetfulness and literacy limitations are discussed. We recommend that diaries should not be used in less educated populations. http://repub.eur.nl/res/pub/8827/ 1998-01-01T00:00:00Z STUDY OBJECTIVE: To assess the validity and accuracy of children's medical utilisation estimates from a health interview and diary and the possible consequences for morbidity estimates. The influence of recall bias and respondent characteristics on the reporting levels was also investigated. DESIGN: Validity study, with the medical record of the general practitioner (GP) as gold standard. In a health interview and three week diary estimates of medical utilisation of children were asked and compared with a GP's medical record. SETTING: General community and primary care centre in the Netherlands. PARTICIPANTS: Parents of 1,805 children and 161 GPs. MAIN RESULTS: The sensitivity of the interview (0.84) is higher than the diary (0.72), while specificity and kappa are higher in the diary (0.96; 0.64) than in the interview (0.91; 0.5-8). Recall bias, expressed as telescoping and heaping, is present in the interview data. Prevalence estimates of all morbidity are much higher in the interview, except for skin problems. Compared with a parental diary more consultations are reported exclusively by the GP for children from ethnic minorities (OR 1.6), jobless (OR 2.3), and less educated mothers (OR 2.6). CONCLUSIONS: Estimates of medical utilisation rates of children are critically influenced by the method of data collection used. Interviews are prone to introduce recall bias, while diaries should only be used in populations with an adequate level of literacy. It is recommended that medical records are used, as they produce most consistent estimates. http://repub.eur.nl/res/pub/8828/ 1998-01-01T00:00:00Z BACKGROUND: Fewer than 20% of all illnesses that occur in the home require the attention of a general practitioner (GP). Whether specific illnesses in children are more likely to need the attention of a GP is poorly understood, as is the influence of various other factors. Health diaries are the most suitable method of collecting comprehensive information about children's health problems at home and in general practice simultaneously. AIM: To investigate the occurrence of, and consultation rates for, specific symptoms in childhood in relation to age, sex, birth order, and place of residence of the child, and season of the year. METHOD: The parents of 1805 children kept a health diary over three weeks and recorded symptoms and consultation behaviour. The symptoms were later combined into illness episodes. RESULTS: Over three weeks, colds/flu (157/1000 children) and respiratory symptoms (114/1000 children) occurred most frequently. More young children (0-4 years) suffered from illness generally. Eleven per cent of all illness episodes required the attention of a GP. Consultation rates differed greatly according to symptoms. A GP was consulted most often for ear (36%) and skin (28%) problems, and least often for headaches (2%) and tiredness (1%). Regardless of symptoms, young children (0-4 years) were taken to a GP twice as often as older children (10-14 years). CONCLUSIONS: This study emphasizes the enormous amount of illness that occurs in children and the fact that more than 80% of all illnesses are dealt with by parents without reference to the professional health care system.