New proposals for the validation of trace-driven simulations

simulation;simulation models;operations research

Modified Normal Demand Distributions in (R,S)-Inventory Models

To model demand, the normal distribution is by far the most popular; the disadvantage that it takes negative values is taken for granted.This paper proposes two modi.cations of the normal distribution, both taking non-negative values only.Safety factors and order-up-to-levels for the familiar (R, S)-control system are derived and compared with the standard values corresponding with the original normal distribution.demand;inventory control

Simulating an (R,s,S) Inventory System

inventory control;simulation models;reorder point;fill rate

Comparison of bias-reducing methods for estimating the parameter in dilution series

Estimation;mathematische statistiek

Exact Fill Rates for (R, s, S) Inventory Control With Gamma Distributed Demand

For the familiar (R; s; S) inventory control system only approximate expressions exist for the fill rate, i.e. the fraction of demand that can be satisfied from stock.Best-known are the approximations derived from renewal theory by Tijms & Groenevelt (1984), holding under specific conditions; in particular, S ¡ s should be reasonably large.They considered, more specifically, the cases of normally and gamma distributed demand.Here, an exact expression for the fill rate is derived, holding generally in the situation that demand has a gamma distribution with known integer-valued parameters, while lead time is constant.This formula is checked through extensive simulations; besides, detailed comparisons are made with Tijms & Groenevelt's approximation.demand;inventory control;simulation

Two-Step Sequential Sampling for Gamma Distributions

estimation bias;extended sampling;sample extension;stochastic sample size;two-step sampling;statistical distribution

Investigating several alternatives for estimating the compound lead time demand in an (s,Q) inventory model

Inventory Models;management science

Multiple-dose design and bias-reducing methods for limiting dilution assays

This paper gives an overview of several (mostly recent) statistical contributions to the theory of Limiting and Serial Dilution Assays (LDA's, SDA's). A simple and useful method is presented for the setup of a design for an LDA or an SDA. This method is based on several user-supplied design parameters, consisting in the researcher's advance information and other parameters inherent to the particular problem. The commonly used Maximum Likelihood (ML) and Minimum Chi-square methods for the estimation of the unknown parameter in an LDA or an SDA are described and compared to several bias-reducing estimation methods, e.g. jackknife and bootstrap versions of the ML method. One particular jackknife version is recommended

How to Determine the Order-up-to Level When Demand is Gamma Distributed with Unknown Parameters

Inventory models need information about the demand distribution. In practice, this information is not known with certainty and has to be estimated with often relatively few historical demand observations. Using these estimates leads to underperformance. This paper focuses on gamma distributed demand and a periodic review, order-up-to inventory control policy, where the order-up-to level satisfies a service equation. Under this policy the underperformance is quantified analytically under strong assumptions and with help of simulation if these assumptions are relaxed. The analytical results can be used to improve the attained service level, such that it approaches the desired service level more closely, even if the assumptions are not met. With help of simulation we show that in some cases this improvement results in reaching the desired service level. For the remaining cases, i.e., the cases in which the desired service level is not reached, the underperformance decreases; improvements range from almost 17% up to over 90%. Moreover, with help of simulation and linear regression further improvements can be obtained. The desired service level is reached in more cases and the underperformance in the other cases is decreased even more compared to using only the first improvement. These improvements range from 57% up to 99% compared to the base case (i.e., do not use analytical results) and from 35% up to over 90% compared to using the analytical results, except for a few cases in which the service hardly improved, but in those cases the attained service level was already very close to the desired one. Finally, the method developed in this paper is applied to real demand data using simulation. The total improvements in this case study range from 53% up to 96%.Unknown Demand Parameters;Inventory Control;Gamma Distribution;Ser- vice Level Criterion;Case Study

Važnost rijeka u oblikovanju izvornim euro-mediteranskih staništa guštera

In pristine conditions, Mediterranean Europe was covered by dense forests during all Pleistocene and in more recent warm periods. Combined with its rather northern position this means that at that time, apart from in coastal environments, lizard habitats would have been present only where open places were formed by the action of storms, natural fires and the erosive forces of rivers. The latter will have been the most reliable and also the most important force, especially combined with the influence of large grazing and browsing animals. Variations in habitat quality in the resulting open spaces are determined by longitudinal gradients in temperature, humidity, steepness and trophic conditions. Present day ecological differentiation and some special characteristics of the lizards will be discussed in the light of these hypotheses.U izvornim uvjetima sredozemna Europa je bila pokrivena gustim šumama tijekom cijelog pleistocena i u novijim toplim periodima. U kombinaciji s njenim više sjevernim položajem to znači da je u to doba, osim u obalnim područjima, staništa guštera moglo biti samo tamo gdje su postojali otvoreni prostori nastali djelovanjem oluja, prirodnih požara i erozivnih djelovanja rijeka. Djelovanje rijeka bilo je najprisutnija i najvažnija sila, posebno u kombinaciji s utjecajem velikih životinja, koje su brstile i pasle. U nastalim otvorenim prostorima varijacije u kvaliteti staništa bile su određene longitudinalnim gradijentima u temperaturi, vlazi, strmini i trofičkim uvjetima. Raspravlja se o sadašnjim ekološkim diferencijacijama i nekim posebnim značajkama guštera u svjetlu iznesenih hipoteza