Estimation of gaussian mixtures with rotationally invariant covariance matrices

Homoscedastic and heteroscedastic Gaussian mixtures differ in the constraints placed on the covariance matrices of the mixture components. A new mixture, called herein a strophoscedastic mixture, is defined by a new constraint, This constraint requires the matrices to be identical under orthogonal trans¬formations, where different transformations are allowed for different matrices. It is shown that the M-step of the EM method for estimating the parameters of strophoscedastic mixtures from sample data is explicitly solvable using singular value decompositions. Consequently, the EM-based maximum likelihood estimation algorithm is as easily implemented for strophoscedastic mixtures as it is for homoscedastic and heteroscedastic mixtures. An example of a “noisy” Archimedian spiral is presented.     

Changes in At-Risk American Men's Crime and Substance Use Trajectories Following Fatherhood

Fatherhood can be a turning point in development and in men's crime and substance use trajectories. At-risk boys (N = 206) were assessed annually from ages 12 to 31 years. Crime, arrest, and tobacco, alcohol, and marijuana use trajectories were examined. Marriage was associated with lower levels of crime and less frequent substance use. Following the birth of a first biological child, men's crime trajectories showed slope decreases, and tobacco and alcohol use trajectories showed level decreases. The older men were when they became fathers, the greater the level decreases were in crime and alcohol use and the less the slope decreases were in tobacco and marijuana use. Patterns are consistent with theories of social control and social timetables.     

The Persistence of Neighboring as a Determinant of Community Attachment: A Community Field Perspective*

We examined the community field perspective as a complement to the linear‐development and systemic models of community attachment, wherein community attachment is defined as a social bond to the community of place. We empirically evaluated indicators of the actor's interaction within the social field, such as the perceived quality of neighboring and density of friendships, using social survey and census data from 99 communities in 1994 and 2004 and evaluated as well the percentage change in these indicators from 1994 to 2004. Results show that the community field perspective can complement previous approaches by highlighting the importance of perceived neighboring and friendship density and the persistence of perceived neighboring over time. We suggest implications for community development efforts aimed at enhancing community attachment.     

Notes on authors

Social network analysis (SNA), a method which can be used to explore networks in various contexts, has received increasing attention. Drawing on the development of European smoke-free policy, this paper explores how a mixed-method approach to SNA can be utilised to investigate a complex policy network. Textual data from public documents, consultation submissions and websites were extracted, converted and analysed using plagiarism detection software and quantitative network analysis and qualitative data from public documents and 35 interviews were thematically analysed. While the quantitative analysis enabled understanding of the network’s structure and components, the qualitative analysis provided in-depth information about specific actors’ positions, relationships and interactions. The paper establishes that SNA is suited to empirically testing and analysing networks in EU policy-making. It contributes to methodological debates about the antagonism between qualitative and quantitative approaches and demonstrates that qualitative and quantitative network analysis can offer a powerful tool for policy analysis.     

Social Movements and Tactical Choice

For much of the past 40 years, the study of social movement tactics has viewed organizers' choices as driven by a desire to maximize efficacy and efficiency within a context of scarce resources and structural constraints. As sociologists increasingly turned toward culture, a new orientation emerged to view tactical choice as a process of gathering, interpreting, and evaluating information within dynamic, uncertain, and often‐contradictory contexts. The importance of the cultural turn has been amply demonstrated in studies of such things as identities, emotions, and collective action frames, but the full implications of its insights continue to be discovered. Four insights in particular warrant greater attention: many core concepts in the study of social movements have an interpretive, subjective, and contingent nature; tactics are a means of communication; social structures are imbued with culture, and culture is thoroughly structured; and social movements sometimes behave irrationally, and what appears to be irrational behavior often is in fact rational. I briefly discuss three areas of scholarship – collective identities, diffusion, and institutional fields – that demonstrate innovative ways that sociologists continue to combine and incorporate these insights and point the way toward a more sophisticated understanding of social movements and tactical choice.     

RELIGIOUS REFERENCE GROUPS AND THE PERSISTENCE OF NORMATIVE BEHAVIOR: AN EMPIRICAL TEST

Reference groups and significant others are vitally important in both the formation and the persistence or change of normative as well as deviant behavior patterns. Thus one's initial religious beliefs and behavior (or lack thereof) reflect the socializing influence of the family. However, the situation may change when young people leave home for education or work, as demonstrated by research that shows decreases in religious beliefs or church attendance when young people leave home to attend college. In contrast to the pattern whereby religiosity declines in a college or university environment, we maintain that students who develop close ties with others who are religious, especially in a highly religious community, will maintain the same patterns of high commitment developed in their families. Specifically, we hypothesize that religious beliefs and participation will be positively related to (1) parents' religious beliefs and practices and (2) current friends' religious beliefs and participation. These hypotheses were tested with a sample of college students living on campus (n = 339). The data support the argument that students' current religious beliefs and behavior are related to both their parents' religiosity and the reinforcing effects of the religiosity of their current friends.