Random Search Model Research Articles

Evaluating random search strategies in three mammals from distinct feeding guilds.

Searching allows animals to find food, mates, shelter and other resources essential for survival and reproduction and is thus among the most important activities performed by animals. Theory predicts that animals will use random search strategies in highly variable and unpredictable environments. Two prominent models have been suggested for animals searching in sparse and heterogeneous environments: (i) the Lévy walk and (ii) the composite correlated random walk (CCRW) and its associated area-restricted search behaviour. Until recently, it was difficult to differentiate between the movement patterns of these two strategies. Using a new method that assesses whether movement patterns are consistent with these two strategies and two other common random search strategies, we investigated the movement behaviour of three species inhabiting sparse northern environments: woodland caribou (Rangifer tarandus caribou), barren-ground grizzly bear (Ursus arctos) and polar bear (Ursus maritimus). These three species vary widely in their diets and thus allow us to contrast the movement patterns of animals from different feeding guilds. Our results showed that although more traditional methods would have found evidence for the Lévy walk for some individuals, a comparison of the Lévy walk to CCRWs showed stronger support for the latter. While a CCRW was the best model for most individuals, there was a range of support for its absolute fit. A CCRW was sufficient to explain the movement of nearly half of herbivorous caribou and a quarter of omnivorous grizzly bears, but was insufficient to explain the movement of all carnivorous polar bears. Strong evidence for CCRW movement patterns suggests that many individuals may use a multiphasic movement strategy rather than one-behaviour strategies such as the Lévy walk. The fact that the best model was insufficient to describe the movement paths of many individuals suggests that some animals living in sparse environments may use strategies that are more complicated than those described by the standard random search models. Thus, our results indicate a need to develop movement models that incorporate factors such as the perceptual and cognitive capacities of animals.

Distributional effects of hiring through networks

How would a policy that bans the use of networks in hiring (e.g. anti-old boy network laws) affect welfare? We answer this question in a random search model in which there are two hiring methods, formal costly channels and referral channels, and there are two types of workers, networked workers, who can be hired through both channels, and non-networked workers, who cannot be hired through referrals. We show that the effect of a referral-restricting policy on non-networked workers can be either positive or negative, depending on model parameters. In our calibration such a policy would make non-networked workers slightly worse off and networked workers substantially worse off.

Currency Exchange in an Open-Economy Random Search Model

AbstractThis paper studies how endogenous currency exchange arises in a two-country, two-currency monetary search model. Although currency exchange is widely observed in the globalized economy, Zhou (1997) is one of the exceptional studies that adopted a search model to generate currency exchange endogenously. Moreover, in her model, currency exchange occurs only in the case where agents rarely consume foreign goods, which is contrary to the well-known fact that international trade increases the number of currency exchanges. We construct a monetary search model that has a feature that increased international trade increases the volume of currency exchange. We develop a two-country model in which each country has two types of agents: local traders who consume only the goods produced in the home country, and international traders who consume only the goods produced in the foreign country. For international traders, the foreign currency gives more opportunity to obtain the foreign goods than the home currency does. Thus, when an international trader holds the home currency, he has an incentive to exchange the home currency for the foreign currency. Unlike Zhou, the model has a feature that currency exchange is more likely to occur when many agents are engaged in international trade.

Unemployment history and frictional wage dispersion

Recent evidence shows that baseline search models struggle to match the observed levels of wage dispersion. This paper studies a random matching search model with human capital losses during unemployment. Wage dispersion increases, as workers accept lower wages to avoid long unemployment spells. The model explains between a third and half of the observed residual wage dispersion. When adding on-the-job search, the model accounts for all of the residual wage dispersion and generates substantial dispersion even for high values of non-market time. The paper thus addresses the trade-off between explaining frictional wage dispersion and the cyclical behavior of unemployment.

Wage posting and business cycles: A quantitative exploration

We provide a quantitative exploration of business cycles in a frictional labor market under contract-posting. The steady-state random search and wage-posting model of Burdett and Mortensen (1998) has become the canonical structural framework for empirical analysis of worker turnover and equilibrium wage dispersion. In this paper, we provide an efficient algorithm to simulate a dynamic stochastic equilibrium version of this model, the Stochastic Burdett–Mortensen model, and evaluate its performance against empirical evidence on fluctuations in unemployment, vacancies and wages.

Minimum prices in a model with search frictions and price posting

In this paper I consider the effects of a minimum price (maximum wage) in a random search model with price (wage) posting. I show that with a minimum price, the price distribution will have a mass point. If the minimum price is sufficiently high, the price distribution will become degenerate.

Efficiency in a search and matching model with participation policy

This note presents a random search and matching model with ex ante heterogeneity in worker productivity. A Social Planner with the power to prevent labor market participation by the least productive is shown to increase economic efficiency. This is true even at the Hosios rule which, under worker homogeneity, implements the constrained efficient allocation.

Display Movement Velocity and Dynamic Visual Search Performance

AbstractIn practice, many visual search tasks are performed under dynamic conditions. An experiment was conducted here to test visual search strategy adopted by a person in a dynamic visual search and to investigate the effects of display movement velocity on search time and detection accuracy in it. Thirty‐five participants were randomly tested with all 10 angular velocities of 0, 2, 4, 6, 8, 10, 12, 14, 16, and 32 deg/s. The data obtained fitted the random search model well. The results revealed that observers utilized a random search strategy during the dynamic visual search process and that display movement velocity influenced search performance. In comparison with static visual search, an angular velocity faster than 4 deg/s resulted in a significant decrement in search performance. The variations of duration of individual fixations, the probability of target detection in a single fixation and visual lobe area were discussed. The obtained relationships between display movement velocity, search time and detection accuracy can serve as a useful guide for designing a dynamic search task, thus helping to maximize the cost‐effectiveness of dynamic search tasks while minimizing errors and misses during the search process.

Search with multi-worker firms

We present a generalization of the standard random-search model of unemployment in which firms hire multiple workers and in which the hiring process is timeconsuming as well as costly. We follow Stole and Zwiebel (1996a, 1996b )a nd assume that wages are determined by continuous bargaining between the firm and its employees. The model generates a nontrivial dispersion of firm sizes; when firms’ production technologies exhibit decreasing returns to labor, it also generates wage dispersion, even when all firms and all workers are ex ante identical. We characterize the steady-state equilibrium and show that, with a suitably chosen distribution of ex ante heterogeneity across firms, it is consistent with several important stylized facts about the joint distribution of firm size, firm growth, and wages in the U.S. economy. We also conduct a numerical investigation of the outof-steady-state dynamics of our model. We find that the responses of unemployment and of the vacancy-to-unemployment ratio to a shock to labor productivity can be somewhat more persistent than in the Mortensen–Pissarides benchmark where each firm employs a single worker.

Cellular self-organization on micro-structured surfaces

Micro-patterned surfaces are frequently used in high-throughput single-cell studies, as they allow one to image isolated cells in defined geometries. Commonly, cells are seeded in excess onto the entire chip, and non-adherent cells are removed from the unpatterned sectors by rinsing. Here, we report on the phenomenon of cellular self-organization, which allows for autonomous positioning of cells on micro-patterned surfaces over time. We prepared substrates with a regular lattice of protein-coated adhesion sites surrounded by PLL-g-PEG passivated areas, and studied the time course of cell ordering. After seeding, cells randomly migrate over the passivated surface until they find and permanently attach to adhesion sites. Efficient cellular self-organization was observed for three commonly used cell lines (HuH7, A549, and MDA-MB-436), with occupancy levels typically reaching 40-60% after 3-5 h. The time required for sorting was found to increase with increasing distance between adhesion sites, and is well described by the time-to-capture in a random-search model. Our approach thus paves the way for automated filling of cell arrays, enabling high-throughput single-cell analysis of cell samples without losses.

Comparing capuchins and coatis: causes and consequences of differing movement ecology in two sympatric mammals

The mechanisms that shape animal movement decisions at the level of an individual or a group of animals can scale up to affect larger-scale ecological processes. Ecologists often use mechanistic animal movement models to understand these links, but animal movement models rarely connect empirically with an understanding of how animals actually decide to move around in their environment. To better understand this relationship, we compared the travel behaviour of two sympatric mammal species that have broadly similar diets: brown capuchin monkeys, Cebus apella nigritus , and ring-tailed coatis, Nasua nasua . According to most mechanistic animal movement models, species that exploit the same resources should show similar movement patterns. Although the fruit component of coati and capuchin diets is very similar, coatis primarily feed on invertebrates in the leaf litter or soil, while capuchins forage on invertebrate prey in the forest canopy. We found that these two species showed markedly different movement patterns: while capuchins typically travelled between fruit trees in relatively straight lines, coatis had significantly more tortuous daily travel paths and frequently visited the same fruit trees more than once per day. These circular coati travel paths would not be predicted by most foraging models. We posit that these differences in coati and capuchin movement patterns are driven by differences in arboreal and terrestrial travel costs, exploitation of fallen fruits and shifts in foraging behaviour over the course of the day. Because these seemingly small differences between the two species lead to major differences in movement behaviour, we posit that animal movement models need to better incorporate (1) travel costs, (2) both directed travel and random food search and (3) realistic diet models that include resources with different nutrient compositions. • We compared travel behaviour between two sympatric mammals with similar diets. • Capuchin monkeys typically showed trapline foraging movements between fruit trees. • Coatis showed circuitous travel and revisited the same areas during the day. • Seemingly minor factors can lead to drastically different movement patterns. • We posit that future animal movement models need to incorporate these factors.

Visual search time in detection tasks with multiple targets: Considering change of the effective stimulus field area

Visual search tasks are of great importance in quality control to ensure that defective products do not get to the marketplace and affect the function, safety and acceptability of products. The experiment reported here was to measure visual search performance for a task involving three targets of identical difficulty. Participants were required to find all three targets. The aims were to see if search times fitted the random strategy model, and particularly, if search times for the second and third targets found would be shorter than those predicted by the current search model for multiple targets. The results showed that for both individual and pooled data the participants tended to use random search strategies thereby validating the random search model for multiple targets. However, search times for locating the second and third targets were shorter than the theoretical times estimated by the model. The reasons for the differences between the actual search times and the theoretical times were analyzed, and target independence in random visual search tasks with multiple targets was discussed. Finally, the concept of the effective stimulus field area and an improved random search performance model for multiple targets were proposed. Relevance to industryThis study investigated visual search performance for three targets and proposed an improved random search performance model for multiple targets. This model can be used to aid determination of effective and efficient stopping policies and to establish search time limits for detection tasks that involve multiple targets.

ОЦіНКА ТОЧНОСТі ЧИСЕЛЬНИХ РОЗРАХУНКіВ НАДіЙНОСТі МОДЕЛЕЙ ФіЛЬТРУВАННЯ ВОЛОГИХ ДИСПЕРСНИХ МАТЕРіАЛіВ

Оцінка точності чисельних розрахунків надійності моделей фільтрування вологих дисперсних матеріалів. Запропоновані і розроблені підходи оцінки точності чисельних розрахунків надійності і ефективності фільтрування дисперсних матеріалів з алгоритмами захисту внутрішньої структури матеріалів від механічного пошкодження.

Dissociation from DNA of Type III Restriction–Modification enzymes during helicase-dependent motion and following endonuclease activity

DNA cleavage by the Type III Restriction–Modification (RM) enzymes requires the binding of a pair of RM enzymes at two distant, inversely orientated recognition sequences followed by helicase-catalysed ATP hydrolysis and long-range communication. Here we addressed the dissociation from DNA of these enzymes at two stages: during long-range communication and following DNA cleavage. First, we demonstrated that a communicating species can be trapped in a DNA domain without a recognition site, with a non-specific DNA association lifetime of ∼200 s. If free DNA ends were present the lifetime became too short to measure, confirming that ends accelerate dissociation. Secondly, we observed that Type III RM enzymes can dissociate upon DNA cleavage and go on to cleave further DNA molecules (they can ‘turnover’, albeit inefficiently). The relationship between the observed cleavage rate and enzyme concentration indicated independent binding of each site and a requirement for simultaneous interaction of at least two enzymes per DNA to achieve cleavage. In light of various mechanisms for helicase-driven motion on DNA, we suggest these results are most consistent with a thermally driven random 1D search model (i.e. ‘DNA sliding’).

Degree distribution and scaling in the connecting-nearest-neighbors model

We present a detailed analysis of the connecting-nearest-neighbors model by Vázquez [Phys. Rev. E 67, 056104 (2003)]. We show that the degree distribution follows a power law, but the scaling exponent can vary with the parameter setting. Moreover, the correspondence of the growing version of the connecting-nearest-neighbors model to the particular random walk model and recursive search model is established.

How Broken DNA Finds Its Template for Repair: A Computational Approach

Homologous recombination (HR) is the process by which a double-strand break in DNA is repaired using an identical donor template. Despite rapid progress in identifying the functions of the proteins that mediate HR, little is known about how broken DNA finds its homologous template. This process, coined homology search, has been difficult to monitor experimentally. Therefore, we present here a computational approach to model the effect of subnuclear positioning and chromatin dynamics on homology search. We found that, in our model, homology search occurs more efficiently if both the cut site and its template are at the nuclear periphery, whereas restricting the movement of the template or the break alone to the periphery markedly increases the time of the search. Immobilization of either component at any position slows down the search. Based on these results, we propose a new model for homology search, the facilitated random search model, which predicts that the search is random, but that nuclear organization and dynamics strongly influence its speed and efficiency.

Validating the Random Search Model for Two Targets of Different Difficulty

A random visual search model was fitted to 1,788 search times obtained from a nonidentical double-target search task. 30 Hong Kong Chinese (13 men, 17 women) ages 18 to 33 years (M = 23, SD = 6.8) took part in the experiment voluntarily. The overall adequacy and prediction accuracy of the model for various search time parameters (mean and median search times and response times) for both individual and pooled data show that search strategy may reasonably be inferred from search time distributions. The results also suggested the general applicability of the random search model for describing the search behavior of a large number of participants performing the type of search used here, as well as the practical feasibility of its application for determination of stopping policy for optimization of an inspection system design. Although the data generally conformed to the model the search for the more difficult target was faster than expected. The more difficult target was usually detected after the easier target and it is suggested that some degree of memory-guided searching may have been used for the second target. Some abnormally long search times were observed and it is possible that these might have been due to the characteristics of visual lobes, nonoptimum interfixation distances and inappropriate overlapping of lobes, as has been previously reported.

Equal-treatment policy in a random search model with taste discrimination

We consider a search model of the labor market with two types of equally productive workers and two types of firms, discriminators and non-discriminators. Without policy intervention, there is wage dispersion between and within the two worker groups, but all wage differences become negligible when the taste for discrimination is small. We analyze the effect of an equal-pay policy, both in combination with affirmative action and without. When equal opportunity of hiring cannot be enforced, wage dispersion increases and wages for minority workers fall substantially relative to laissez faire. Sometimes also the wage gap between worker groups widens in response to the policy.

Block Recursive Equilibria for Stochastic Models of Search on the Job

In this paper, we develop a general stochastic model of directed search on the job. Like in the analogous models of random search on the job, the state of the economy in our model includes the infinite-dimensional distribution of workers across different employment states (unemployment, and employment at different wages). Unlike the analogous models of random search on the job, our model admits an equilibrium in which the agents' value and policy functions does not depend on the distribution of workers. We refer to this type of equilibrium as a Block Recursive Equilibrium. Therefore, while solving the equilibrium of a random search model in a stochastic environment is a difficult task both analytically and computationally, solving the Block Recursive Equilibrium of our model is as easy as solving a representativeagent model.

Equal-Treatment Policy in a Random Search Model with Taste Discrimination

We consider a search model of the labor market with two types of equally productive workers and two types of firms, discriminators and non-discriminators. Without policy intervention, there is wage dispersion between and within the two worker groups, but all wage differences become negligible when the taste for discrimination is small. We analyze the effect of an equal-pay policy, both in combination with affirmative action and without. When equal opportunity of hiring cannot be enforced, wage dispersion increases and wages for minority workers fall substantially relative to laissez faire. Sometimes also the wage gap between worker groups widens in response to the policy.