Correcting mean-field approximations for birth-death-movement processes

On the microscale, migration, proliferation and death are crucial in the development, homeostasis and repair of an organism; on the macroscale, such effects are important in the sustainability of a population in its environment. Dependent on the relative rates of migration, proliferation and death, spatial heterogeneity may arise within an initially uniform field; this leads to the formation of spatial correlations and can have a negative impact upon population growth. Usually, such effects are neglected in modeling studies and simple phenomenological descriptions, such as the logistic model, are used to model population growth. In this work we outline some methods for analyzing exclusion processes which include agent proliferation, death and motility in two and three spatial dimensions with spatially homogeneous initial conditions. The mean-field description for these types of processes is of logistic form; we show that, under certain parameter conditions, such systems may display large deviations from the mean field, and suggest computationally tractable methods to correct the logistic-type description.

Distinguishing between mean-field, moment dynamics and stochastic descriptions of birth-death-movement processes

Mathematical descriptions of birth–death–movement processes are often calibrated to measurements from cell biology experiments to quantify tissue growth rates. Here we describe and analyze a discrete model of a birth–death-movement process applied to a typical two–dimensional cell biology experiment. We present three different descriptions of the system: (i) a standard mean–field description which neglects correlation effects and clustering; (ii) a moment dynamics description which approximately incorporates correlation and clustering effects, and; (iii) averaged data from repeated discrete simulations which directly incorporates correlation and clustering effects. Comparing these three descriptions indicates that the mean–field and moment dynamics approaches are valid only for certain parameter regimes, and that both these descriptions fail to make accurate predictions of the system for sufficiently fast birth and death rates where the effects of spatial correlations and clustering are sufficiently strong. Without any method to distinguish between the parameter regimes where these three descriptions are valid, it is possible that either the mean–field or moment dynamics model could be calibrated to experimental data under inappropriate conditions, leading to errors in parameter estimation. In this work we demonstrate that a simple measurement of agent clustering and correlation, based on coordination number data, provides an indirect measure of agent correlation and clustering effects, and can therefore be used to make a distinction between the validity of the different descriptions of the birth–death–movement process.

Epistemological lens : birth, death and ecology of documentary practices

This paper considers the epistemological life cycle of the camera lens in documentary practices. The 19th century industrial economies that manufactured and commercialised the camera lens have engendered political and economic contingencies on documentary practices to sustain a hegemonic and singular interpretive epistemology. Colonial documentary practices are considered from the viewpoint of manipulative hegemonic practices - all of which use the interpretive epistemology of the camera lens to capitalise a viewpoint which is singular and possesses the power to sustain its own status and economic privilege. I suggest that decolonising documentary practices can be nurtured in what Boaventura de Sousa Santos proposes as an 'ecology of knowledges' (Andreotti, Ahenakew, & Cooper 2011) - a way of including the epistemologies of cultures beyond the 'abyssal' (Santos), outside the limits of epistemological dominance. If an 'epistemicide' (Santos) of indigenous knowledges in the dominant limits has occurred then in an ecology of knowledges the limits become limitless and what were once invisible knowledges, come into their own ontological and epistemological being: as free agents and on their own terms. In an ecology of knowledges, ignorance and blindness may still exist but are not privileged. The decolonisation of documentary practices inevitably destabilises prevailing historicities and initiates ways for equal privilege to exist between multiple epistemologies.

Incorporating spatial correlations into multispecies mean-field models

In biology, we frequently observe different species existing within the same environment. For example, there are many cell types in a tumour, or different animal species may occupy a given habitat. In modelling interactions between such species, we often make use of the mean field approximation, whereby spatial correlations between the locations of individuals are neglected. Whilst this approximation holds in certain situations, this is not always the case, and care must be taken to ensure the mean field approximation is only used in appropriate settings. In circumstances where the mean field approximation is unsuitable we need to include information on the spatial distributions of individuals, which is not a simple task. In this paper we provide a method that overcomes many of the failures of the mean field approximation for an on-lattice volume-excluding birth-death-movement process with multiple species. We explicitly take into account spatial information on the distribution of individuals by including partial differential equation descriptions of lattice site occupancy correlations. We demonstrate how to derive these equations for the multi-species case, and show results specific to a two-species problem. We compare averaged discrete results to both the mean field approximation and our improved method which incorporates spatial correlations. We note that the mean field approximation fails dramatically in some cases, predicting very different behaviour from that seen upon averaging multiple realisations of the discrete system. In contrast, our improved method provides excellent agreement with the averaged discrete behaviour in all cases, thus providing a more reliable modelling framework. Furthermore, our method is tractable as the resulting partial differential equations can be solved efficiently using standard numerical techniques.

An investigation into the barriers to the implementation of automation and robotics technologies in the construction industry

The rising problems associated with construction such as decreasing quality and productivity, labour shortages, occupational safety, and inferior working conditions have opened the possibility of more revolutionary solutions within the industry. One prospective option is in the implementation of innovative technologies such as automation and robotics, which has the potential to improve the industry in terms of productivity, safety and quality. The construction work site could, theoretically, be contained in a safer environment, with more efficient execution of the work, greater consistency of the outcome and higher level of control over the production process. By identifying the barriers to construction automation and robotics implementation in construction, and investigating ways in which to overcome them, contributions could be made in terms of better understanding and facilitating, where relevant, greater use of these technologies in the construction industry so as to promote its efficiency. This research aims to ascertain and explain the barriers to construction automation and robotics implementation by exploring and establishing the relationship between characteristics of the construction industry and attributes of existing construction automation and robotics technologies to level of usage and implementation in three selected countries; Japan, Australia and Malaysia. These three countries were chosen as their construction industry characteristics provide contrast in terms of culture, gross domestic product, technology application, organisational structure and labour policies. This research uses a mixed method approach of gathering data, both quantitative and qualitative, by employing a questionnaire survey and an interview schedule; using a wide range of sample from management through to on-site users, working in a range of small (less than AUD0.2million) to large companies (more than AUD500million), and involved in a broad range of business types and construction sectors. Detailed quantitative (statistical) and qualitative (content) data analysis is performed to provide a set of descriptions, relationships, and differences. The statistical tests selected for use include cross-tabulations, bivariate and multivariate analysis for investigating possible relationships between variables; and Kruskal-Wallis and Mann Whitney U test of independent samples for hypothesis testing and inferring the research sample to the construction industry population. Findings and conclusions arising from the research work which include the ranking schemes produced for four key areas of, the construction attributes on level of usage; barrier variables; differing levels of usage between countries; and future trends, have established a number of potential areas that could impact the level of implementation both globally and for individual countries.

Investigation of mediated oxidation of ascorbic acid by ferrocenemethanol using large-amplitude fourier transformed ac voltammetry under quasi-reversible electron-transfer conditions at an indium tin oxide electrode

The ability of the technique of large-amplitude Fourier transformed (FT) ac voltammetry to facilitate the quantitative evaluation of electrode processes involving electron transfer and catalytically coupled chemical reactions has been evaluated. Predictions derived on the basis of detailed simulations imply that the rate of electron transfer is crucial, as confirmed by studies on the ferrocenemethanol (FcMeOH)-mediated electrocatalytic oxidation of ascorbic acid. Thus, at glassy carbon, gold, and boron-doped diamond electrodes, the introduction of the coupled electrocatalytic reaction, while producing significantly enhanced dc currents, does not affect the ac harmonics. This outcome is as expected if the FcMeOH (0/+) process remains fully reversible in the presence of ascorbic acid. In contrast, the ac harmonic components available from FT-ac voltammetry are predicted to be highly sensitive to the homogeneous kinetics when an electrocatalytic reaction is coupled to a quasi-reversible electron-transfer process. The required quasi-reversible scenario is available at an indium tin oxide electrode. Consequently, reversible potential, heterogeneous charge-transfer rate constant, and charge-transfer coefficient values of 0.19 V vs Ag/AgCl, 0.006 cm s (-1) and 0.55, respectively, along with a second-order homogeneous chemical rate constant of 2500 M (-1) s (-1) for the rate-determining step in the catalytic reaction were determined by comparison of simulated responses and experimental voltammograms derived from the dc and first to fourth ac harmonic components generated at an indium tin oxide electrode. The theoretical concepts derived for large-amplitude FT ac voltammetry are believed to be applicable to a wide range of important solution-based mediated electrocatalytic reactions.

Investigating death: the emotional and cultural challenges for police

The over-representation of vulnerable populations within the criminal justice system, and the role of police in perpetuating this, has long been a topic of discussion in criminology. What is less discussed is the way in which non-criminal investigations by police, in areas like a death investigation, may similarly disadvantage and discriminate against vulnerable populations. In Australia, as elsewhere, it is police who are responsible for investigating both suspicious and violent deaths like homicide as well as non-suspicious, violent deaths like accidents and suicides. Police are also the agents tasked with investigating deaths which are neither violent nor suspicious but occur outside hospitals and other care facilities. This paper, part of a larger funded Australian research project focusing on the ways in which cultural and religious differences are dealt with during the death investigation process, reports on how police describe – or are described by others – during their role in a non-suspicious death investigation, and the challenges that such investigations raise for police and policing. The employment of police liaison officers is discussed as one response to the difficulty of policing cultural and religious difference with variable results.

Translation and news making : a study of contemporary arabic television

Arabic satellite television has recently attracted tremendous attention in both the academic and professional worlds, with a special interest in Aljazeera as a curious phenomenon in the Arab region. Having made a household name for itself worldwide with the airing of the Bin Laden tapes, Aljazeera has set out to deliberately change the culture of Arabic journalism, as it has been repeatedly stated by its current General Manager Waddah Khanfar, and to shake up the Arab society by raising awareness to issues never discussed on television before and challenging long-established social and cultural values and norms while promoting, as it claims, Arab issues from a presumably Arab perspective. Working within the meta-frame of democracy, this Qatari-based network station has been received with mixed reactions ranging from complete support to utter rejection in both the west and the Arab world. This research examines the social semiotics of Arabic television and the socio-cultural impact of translation-mediated news in Arabic satellite television, with the aim to carry out a qualitative content analysis, informed by framing theory, critical linguistic analysis, social semiotics and translation theory, within a re-mediation framework which rests on the assumption that a medium “appropriates the techniques, forms and social significance of other media and attempts to rival or refashion them in the name of the real" (Bolter and Grusin, 2000: 66). This is a multilayered research into how translation operates at two different yet interwoven levels: translation proper, that is the rendition of discourse from one language into another at the text level, and translation as a broader process of interpretation of social behaviour that is driven by linguistic and cultural forms of another medium resulting in new social signs generated from source meaning reproduced as target meaning that is bound to be different in many respects. The research primarily focuses on the news media, news making and reporting at Arabic satellite television and looks at translation as a reframing process of news stories in terms of content and cultural values. This notion is based on the premise that by its very nature, news reporting is a framing process, which involves a reconstruction of reality into actualities in presenting the news and providing the context for it. In other words, the mediation of perceived reality through a media form, such as television, actually modifies the mind’s ordering and internal representation of the reality that is presented. The research examines the process of reframing through translation news already framed or actualized in another language and argues that in submitting framed news reports to the translation process several alterations take place, driven by the linguistic and cultural constraints and shaped by the context in which the content is presented. These alterations, which involve recontextualizations, may be intentional or unintentional, motivated or unmotivated. Generally, they are the product of lack of awareness of the dynamics and intricacies of turning a message from one language form into another. More specifically, they are the result of a synthesis process that consciously or subconsciously conforms to editorial policy and cultural interpretive frameworks. In either case, the original message is reproduced and the news is reframed. For the case study, this research examines news broadcasts by the now world-renowned Arabic satellite television station Aljazeera, and to a lesser extent the Lebanese Broadcasting Corporation (LBC) and Al- Arabiya where access is feasible, for comparison and crosschecking purposes. As a new phenomenon in the Arab world, Arabic satellite television, especially 24-hour news and current affairs, provides an interesting area worthy of study, not only for its immediate socio-cultural and professional and ethical implications for the Arabic media in particular, but also for news and current affairs production in the western media that rely on foreign language sources and translation mediation for international stories.

Panel studies of new venture creation : a review and suggestions for future research

We review all journal articles based on “PSED-type” research, i.e., longitudinal, empirical studies of large probability samples of on-going, business start-up efforts. We conclude that the research stream has yielded interesting findings; sometimes by confirming prior research with a less bias-prone methodology and at other times by challenging whether prior conclusions are valid for the early stages of venture development. Most importantly, the research has addressed new, process-related research questions that prior research has shunned or been unable to study in a rigorous manner. The research has revealed an enormous and fascinating variability in new venture creation that also makes it challenging to arrive at broadly valid generalizations. An analysis of the findings across studies as well as an examination of those studies that have been relatively more successful at explaining outcomes give good guidance regarding what is required in order to achieve strong and credible results. We compile and present such advice to users of existing data sets and designers of new projects in the following areas: Statistically representative and/or theoretically relevant sampling; Level of analysis issues; Dealing with process heterogeneity; Dealing with other heterogeneity issues, and Choice and interpretation of dependent variables.

The measurement of information system use: preliminary considerations

The concept of system use has suffered from a "too simplistic definition" (DeLone and McLean [9], p. 16). This paper reviews various attempts at conceptualization and measurement of system use and then proposes a re-conceptualization of it as "the level of incorporation of an information system within a user's processes." We then go on to develop the concept of a Functional Interface Point and four dimensions of system usage: automation level, the proportion of the business process encoded by the information system; extent, the proportion of the FIPs used by the business process; frequency, the rate at which FIPs are used by the participants in the process; and thoroughness, the level of use of information/functionality provided by the system at an FIP. The article concludes with a discussion of some implications of this re-conceptualization and areas for follow on research.

The Measurement of Information System Use: Preliminary Considerations

DeLone and McLean (1992, p. 16) argue that the concept of “system use” has suffered from a “too simplistic definition.” Despite decades of substantial research on system use, the concept is yet to receive strong theoretical scrutiny. Many measures of system use and the development of measures have been often idiosyncratic and lack credibility or comparability. This paper reviews various attempts at conceptualization and measurement of system use and then proposes a re-conceptualization of it as “the level of incorporation of an information system within a user’s processes.” The definition is supported with the theory of work systems, system, and Key-User-Group considerations. We then go on to develop the concept of a Functional- Interface-Point (FIP) and four dimensions of system usage: extent, the proportion of the FIPs used by the business process; frequency, the rate at which FIPs are used by the participants in the process; thoroughness, the level of use of information/functionality provided by the system at an FIP; and attitude towards use, a set of measures that assess the level of comfort, degree of respect and the challenges set forth by the system. The paper argues that the automation level, the proportion of the business process encoded by the information system has a mediating impact on system use. The article concludes with a discussion of some implications of this re-conceptualization and areas for follow on research.

Simplified method for including spatial correlations in mean-field approximations

Biological systems involving proliferation, migration and death are observed across all scales. For example, they govern cellular processes such as wound-healing, as well as the population dynamics of groups of organisms. In this paper, we provide a simplified method for correcting mean-field approximations of volume-excluding birth-death-movement processes on a regular lattice. An initially uniform distribution of agents on the lattice may give rise to spatial heterogeneity, depending on the relative rates of proliferation, migration and death. Many frameworks chosen to model these systems neglect spatial correlations, which can lead to inaccurate predictions of their behaviour. For example, the logistic model is frequently chosen, which is the mean-field approximation in this case. This mean-field description can be corrected by including a system of ordinary differential equations for pair-wise correlations between lattice site occupancies at various lattice distances. In this work we discuss difficulties with this method and provide a simplication, in the form of a partial differential equation description for the evolution of pair-wise spatial correlations over time. We test our simplified model against the more complex corrected mean-field model, finding excellent agreement. We show how our model successfully predicts system behaviour in regions where the mean-field approximation shows large discrepancies. Additionally, we investigate regions of parameter space where migration is reduced relative to proliferation, which has not been examined in detail before, and our method is successful at correcting the deviations observed in the mean-field model in these parameter regimes.

When to pull the plug and when to take the plunge : timing strategic decisions about new ventures

It is not uncommon for firms to explore a new venture under the belief it will generate profits, only to find out later that although costs accumulated, profits did not materialize. To manage the high level of uncertainty involved in this process, new ventures are generally designed as vehicles of exploration (Wu, 2012) that allow for a staged investment of resources, starting with small initial investments that can be scaled up or discontinued as uncertainty is resolved over time (Folta, 1998; Li and Chi, 2013). As such, new ventures provide firms a vehicle by which they can probe an uncertain future (Brown and Eisenhardt, 1997) without fully committing early on to an irreversible course of action (Folta, Johnson, and O’Brien, 2006). Our focus in the present paper is on the timing of strategic decisions that firms make regarding their exploration ventures. Prior research in the fields of entrepreneurship, real options reasoning, and decision speed has demonstrated a link between the timing of making decisions and performance (Baum and Wally, 2003; Eisenhardt, 1989; Judge and Miller, 1991). The antecedents to the timing of decisions, however, are less understood and pose an interesting dilemma.

Large Scale Simulations of Hippocampal-Neocortical Interactions in a Parallel Version of Genesis

A hippocampal-CA3 memory model was constructed with PGENESIS, a recently developed version of GENESIS that allows for distributed processing of a neural network simulation. A number of neural models of the human memory system have identified the CA3 region of the hippocampus as storing the declarative memory trace. However, computational models designed to assess the viability of the putative mechanisms of storage and retrieval have generally been too abstract to allow comparison with empirical data. Recent experimental evidence has shown that selective knock-out of NMDA receptors in the CA1 of mice leads to reduced stability of firing specificity in place cells. Here a similar reduction of stability of input specificity is demonstrated in a biologically plausible neural network model of the CA3 region, under conditions of Hebbian synaptic plasticity versus an absence of plasticity. The CA3 region is also commonly associated with seizure activity. Further simulations of the same model tested the response to continuously repeating versus randomized nonrepeating input patterns. Each paradigm delivered input of equal intensity and duration. Non-repeating input patterns elicited a greater pyramidal cell spike count. This suggests that repetitive versus non-repeating neocortical inpus has a quantitatively different effect on the hippocampus. This may be relevant to the production of independent epileptogenic zones and the process of encoding new memories.