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In dit proefschrift worden drie verschillende studies beschreven naar de socialisatie en selectie van leeftijdsgenoten. Hierbij is onderzocht op basis van welke kenmerken leerlingen elkaar selecteren als vrienden of groepsgenoten, en op welke kenmerken ze elkaar socialiseren. Voor het onderzoek hebben we sociale netwerk technieken (ERGM en SIENA) gebruikt. Uit de resultaten van de eerste studie (bij 296 basisschoolleerlingen uit groep 8) bleek dat etnische achtergrond van leerlingen een rol speelt bij de selectie van zowel vrienden als 'sociale interactie partners', die niet per sé tot de vrienden horen. Uit de andere twee studies (bij 542 middelbare schoolleerlingen uit 2e klassen ) bleek dat leerlingen elkaar niet selecteren als vrienden op basis van schoolprestaties. Ze bleken elkaar daarentegen wel te beïnvloeden, als het gaat om prestaties op de vakken Engels en Nederlands, en niet als het gaat om prestaties voor wiskunde. Vrienden werden tevens niet geselecteerd op basis van internaliserend probleemgedrag, maar wel op basis van externaliserend probleemgedrag: leerlingen met vergelijkbaar probleemgedrag zochten elkaar op als vrienden. Leerlingen trokken zich niet terug en werden niet vermeden op basis van hun probleemgedrag, en socialiserden elkaar niet m.b.t. internaliserend probleemgedrag. Wel bleken vrienden elkaar te socialiseren in termen van externaliserend probleemgedrag.

Adolescent alcohol use remains an important public health concern. One of the most salient and consistent predictors for drinking behaviour among young people is peer influence. A systematic review of longitudinal studies that examined the effect of peer influence on adolescent alcohol use between January 1997 and February 2011 is presented. Twenty-two studies fulfilled inclusion criteria and were reviewed. All but one study confirmed affiliation with alcohol-using or deviant peers as prospective predictors for the development of adolescent alcohol use. Findings revealed that existing longitudinal studies that have used multivariate analytic techniques to segregate peer influence (whereby adolescents start drinking after exposure to alcohol-using friends) and peer selection (whereby adolescents that start drinking without alcohol-using friends subsequently seek out drinking peers) effects consistently report significant peer influence effects. However, studies are unable to elucidate the relative contribution and developmental sequence of peer influence and selection. Existing research is synthesised to model the developmental influence of peer processes on adolescent alcohol use. Future research directions are recommended to inform better designed investigations that can lead to more effective endeavours to address peer processes in prevention efforts.

This paper investigates strategic monitoring behavior within group lending. We show that monitoring efforts of group members differ in equilibrium due to the asymmetry between members in terms of future profits. In particular, we show that the entrepreneur with the highest future profits also puts in the highest monitoring effort. Moreover, monitoring efforts differ between group members due to free-riding: one member reduces her level of monitoring if the other increases her monitoring effort. This effect is also at play when we introduce a group leader into the model. The individual who becomes the group leader supplies more monitoring effort than in the benchmark case. We empirically test the model using data from a survey of microfinance in Eritrea and show that the group leader attaches more weight to future periods than nonleaders in the group, which may explain why a large part of total monitoring is done by the leader.

Aims The main goal of this study was to examine differences between adolescent male and female friendship networks regarding smoking-based selection and influence processes using newly developed social network analysis methods that allow the current state of continuously changing friendship networks to act as a dynamic constraint for changes in smoking behaviour, while allowing current smoking behaviour to be simultaneously a dynamic constraint for changes in friendship networks. Design Longitudinal design with four measurements. Setting Nine junior high schools in Finland. Participants A total of 1163 adolescents (mean age = 13.6 years) who participated in the control group of the ESFA (European Smoking prevention Framework Approach) study, including 605 males and 558 females. Measurements Smoking behaviour of adolescents, parents, siblings and friendship ties. Findings Smoking-based selection of friends was found in male as well as female networks. However, support for influence among friends was found only in female networks. Furthermore, females and males were both influenced by parental smoking behaviour. Conclusions In Finnish adolescents, both male and female smokers tend to select other smokers as friends but it appears that only females are influenced to smoke by their peer group. This suggests that prevention campaigns targeting resisting peer pressure may be more effective in adolescent girls than boys.

This study explores the determinants of popularity within friendship and advice networks. We involved almost 200 college students in an experiment to predict how personality traits, self-monitoring, creativity, intelligence, energy, and beauty influence the development of friendship and advice networks. Our results indicate that physical attractiveness is a key to develop both friendship and task-related interactions, whereas perceived intelligence and creativity play an important role in the advice network. Our findings seem to support the idea that there might be a kernel of truth in the stereotype that attractiveness correlates with positive social traits and successful outcomes.

Theoretical models on group lending assume the formation of groups of homogenous risk types. Recent theoretical and empirical findings challenge this view arguing that when markets for insurance are missing, risk homogeneity may not hold any more and risk heterogeneity can be the optimal outcome. Using data from an MFI in Tigray (Ethiopia), this article examines the homogeneity hypothesis and reflects on implications for repayment. No evidence is found that supports risk homogeneity, even accounting for matching frictions. However, we also do not find an explicit link between the presence of risk heterogeneity and side-payments due to missing insurance as suggested in the literature. Instead, other trust-based social networks seem to underlie heterogeneity. Such social networks are often synchronized with credit groups and influence the probability of repayment under heterogeneity. The implication is that successful repayment rates in group lending need not arise only under risk homogeneity but can also arise under risk heterogeneity. Heterogeneity may also serve to bridge missing insurance markets in poor rural environments. MFIs therefore need to consider such local conditions when designing their lending schemes

Microfinance has received a lot of attention recently, both from policy makers as well as in academic circles. Two of the main topics that have been hotly debated are explaining joint liability group lending and its implications for reducing information asymmetries, and the trade-off between the financial sustainability and outreach of microfinance programmes. This Feature contains three novel empirical contributions providing new insights with respect to why and how joint liability group lending works. It also contains the first large-scale systematic analysis of the trade-off between financial performance and outreach of microfinance institutions.

Although stochastic actor based models (e.g., as implemented in the SIENA software program) are growing in popularity as a technique for estimating longitudinal network data, a relatively understudied issue is the consequence of missing network data for longitudinal analysis. We explore this issue in our research note by utilizing data from four schools in an existing dataset (the AddHealth dataset) over three time points, assessing the substantive consequences of using four different strategies for addressing missing network data. The results indicate that whereas some measures in such models are estimated relatively robustly regardless of the strategy chosen for addressing missing network data, some of the substantive conclusions will differ based on the missing data strategy chosen. These results have important implications for this burgeoning applied research area, implying that researchers should more carefully consider how they address missing data when estimating such models.

This paper proposes two novel optimized BitTorrent-like protocols for interactive multimedia streaming: the Simple Interactive Streaming Protocol (SISP) and the Exclusive Interactive Streaming Protocol (EISP). The former chiefly seeks a trade-off between playback continuity and data diversity, while the latter is mostly focused on playback continuity. To assure a thorough and up-to-date approach, related work is carefully examined and important open issues, concerning the design of BitTorrent-like algorithms, are analyzed as well. Through simulations, in a variety of near-real file replication scenarios, the novel protocols are evaluated using distinct performance metrics. Among the major findings, the final results show that the two novel proposals are efficient and, besides, focusing on playback continuity ends up being the best design concept to achieve high quality of service. Lastly, avenues for further research are included at the end of this paper as well., 20 pages

The current study examined the development of bullying and defending over a 1-year period as related to friends' influence and individual and friends' moral disengagement (i.e., self-justification mechanisms that allow one to avoid moral self-censure of transgressive actions) in children and young adolescents. Via longitudinal social network analysis (RSiena), it was tested whether similarity between friends in bullying and defending developed over time due to friends' influence, while controlling for friendship selection processes, and whether there were differences in these processes between children (age 9-10 years; n = 133; 42.9% girls) and young adolescents (age 11-14 years; n = 236; 40.6% girls). Results showed that individuals selected peers as friends who were similar in bullying and became more similar to friends in bullying over time, but only in early adolescence. Moreover, there was marginal support that friends' influence was stronger in young adolescents with higher moral disengagement. In early adolescence, bullying was also indirectly influenced through friends' moral disengagement, with different effects for boys and girls. With regard to defending, young adolescents maintained friendships with peers who were similar in defending, and became more similar to friends in terms of defending over time. These findings suggest important differences between late childhood and early adolescence in socialization processes and indicate that in early adolescence, friends' influence on the development of bullying is partially affected by moral disengagement. (PsycINFO Database Record (c) 2014 APA, all rights reserved).

International audience; Television transmitted over IP (IPTV) presents numerous opportunities for users as well as service providers, and has attracted significant interest from industry as well as research communities in recent years. Among the emerging IPTV delivery architectures, the peer-to-peer based delivery mechanism is considered attractive due to the relative ease of service deployment and potential bandwidth savings. However, the question of how well P2PTV networks would support a growing number of users has not been fully investigated so far. In this paper, we try to address this question by studying scalability and efficiency factors in a typical P2P based live streaming network. Through the use of the data provided by a production P2PTV network, we carry out simulations whose results show that there are still hurdles to overcome before P2P based live streaming could become widely deployed.

Due to its ability to deliver scalable and fault-tolerant solutions, applications based on the peer-to-peer (P2P) paradigm are used by millions of users on the internet. Recently, wireless mesh networks (WMNs) have attracted a lot of interest from both academia and industry, because of their potential to provide flexible and alternative broadband wireless internet connectivity. However, due to various reasons such as unstable wireless link characteristics and multi-hop forwarding operation, the performance of current P2P systems is rather low in WMNs. This dissertation studies the technological challenges involved while deploying P2P systems over WMNs. We study the benefits of location-awareness and resource replication to the P2P overlay while targeting efficient resource lookup in WMNs. We further propose a cross-layer information exchange between the P2P overlay and the WMN in order to reduce resource lookup delay by augmenting the overlay routing table with physical neighborhood and resource lookup history information. Aiming to achieve throughput maximization and fairness in P2P systems, we model the peer selection problem as a mathematical optimization problem by using a set of mixed integer linear equations. A study of the model reveals the relationship between peer selection, resource replication and channel assignment on the performance of P2P systems over WMNs. We extend the model by formulating the P2P download problem as chunk scheduling problem. As a novelty, we introduce constraints to model the capacity limitations of the network due to the given routing and channel assignment strategy. Based on the analysis of the model, we propose a new peer selection algorithm which incorporates network load information and multi-path routing capability. By conducting testbed experiments, we evaluate the achievable throughput in multi-channel multi-radio WMNs. We show that the adjacent channel interference (ACI) problem in multi-radio systems can be mitigated, making better use of the available spectrum. Important lessons learned are also outlined in order to design practical channel and channel bandwidth assignment algorithms in multi-channel multi-radio WMNs.

Stochastic actor-based models are models for network dynamics that can represent a wide variety of influences on network change, and allow to estimate parameters expressing such influences, and test corresponding hypotheses. The nodes in the network represent social actors, and the collection of ties represents a social relation. The assumptions posit that the network evolves as a stochastic process 'driven by the actors', i.e., the model lends itself especially for representing theories about how actors change their outgoing ties. The probabilities of tie changes are in part endogenously determined, i.e., as a function of the current network structure itself, and in part exogenously, as a function of characteristics of the nodes ('actor covariates') and of characteristics of pairs of nodes ('dyadic covariates'). In an extended form, stochastic actor-based models can be used to analyze longitudinal data on social networks jointly with changing attributes of the actors: dynamics of networks and behavior.This paper gives an introduction to stochastic actor-based models for dynamics of directed networks, using only a minimum of mathematics. The focus is on understanding the basic principles of the model, understanding the results, and on sensible rules for Model selection. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.

We introduce in this paper the notion of application-aware optimal peer selection. With the advent of P2P and overlay networks, many applications of our days need to select the best peer to contact, either to transfer some data or to be positioned in an overlay network. This selection is still with no clear solution given the heterogeneity of the Internet in terms of path characteristics and access link speed, and the diversity of application requirements. Most of existing protocols rely on simple heuristics as for example choosing the closest peer in terms of delay. We believe that the selection of the best peer should be a function of more parameters (delay, bandwidth, loss rate) and subject to some application utility function (e.g., delay vs. bandwidth). This work aims at motivating the need for taking application requirements coupled with network path measurements into account during the peer selection process. The work consists of running extensive measurements over the PlanetLab overlay network and comparing different peer selection policies. One observation made in this work is that over PlanetLab, the best peer to contact is not always the closest one and it changes with the application needs. Another observation is that path characteristics are not strongly correlated with each other (e.g., smaller delay does not always mean larger bandwidth) which makes their separate use insufficient.

A snowball sampling procedure is simulated by computer. In snowball sampling, a small first-round sample of respondents, in this case editors of journals in several specialities, are asked to name second-round respondents to be contacted for a similar request for a third round, etc. In our survey, respondents were asked to nominate peer scientists for their contributions and expertise in their specialty. We used the simulation to estimate the effect of the number of rounds on the fraction of experts likely to be named, and to investigate the effect of other parameters. We found that it would take many rounds before every expert in a specialty is nominated. The simulation considered the effect of specialty subdivisions and showed how the distribution changes as the sample increases.

Understanding the key factors that influence smoking behavior, especially during adolescence, has a meaningful impact on public health. This study examined the impact of parent modelling, peer influence and peer selection on adolescent smoking behavior in two Portuguese cohorts followed for three years.A questionnaire was delivered in classes and schools randomly selected, three times, one per year (cohort1: time1-7th, time2-8th, time3-9th; cohort2: time1-10th, time2-11th, time3-12th graders).The sample included a total of 656 students (402 younger [time1 Mage = 13.17, SD = 0.53, 63.7% girls;] and 254 older [time 1 Mage = 16.20, SD = 0.53, 65% girls)).Longitudinal data were examined through an autoregressive cross-lagged model (ARCL). The model explained 35% of the variance in smoking behavior at T3 for the global sample (4% for the younger and 58% for the older).Over time, in both cohorts, the percentage of never smokers decreased sharply and the percentage of regular smokers increased rapidly. We observed that participants in the older cohort had higher chances of smoking if their parents smoked. Nevertheless, we did not find a parental modelling effect in the longitudinal model. Peer influence and peer selection influenced smoking behavior. However, peer selection influenced the youngest group, both processes influenced the middle age group, and only peer influence influenced the oldest. Best friend and friends had a stronger impact on the younger while friends and same grade students had a stronger impact on the older. Prevention programs should regard these differences of interpersonal influences through adolescent development and specific strategies for different age groups should be considered.