Domains of convergence for the EM algorithm: a cautionary tale in a location estimation problem

The EM algorithm is a popular method for maximizing a likelihood in the presence of incomplete data. When the likelihood has multiple local maxima, the parameter space can be partitioned into domains of convergence, one for each local maximum. In this paper we investigate these domains for the location family generated by the t-distribution. We show that, perhaps somewhat surprisingly, these domains need not be connected sets. As an extreme case we give an example of a domain which consists of an infinite union of disjoint open intervals. Thus the convergence behaviour of the EM algorithm can be quite sensitive to the starting point.     

Are mating strategies and mating tactics independent constructs?

This study explored the constructs of mating tactics and mating strategy. These constructs are conceptually related but distinct. In current research, the measurement of one of these constructs is often viewed as being indicative of the other. Therefore, an exploration of these constructs will enhance understanding of study outcomes in this research area. Self-report measures of mating tactics and strategies were administered to 183 female participants, aged 18-45 years. The Escalating Sexual Encounters Questionnaire (ESEQ, Greer & Buss, 1994), the Derogatis' Sexual Experience Scale (Derogatis & Melisaratos, 1979), the Sexual Strategies Measure (SSM, Schmitt, 1996), the Sociosexual Orientation Inventory (Simpson & Gangestad, 1991), and two questions assessing age at menarche and total number of sexual partners were administered. Exploratory factor analysis with oblique rotation produced two distinct factors reflecting a "tactic"-based factor and a "strategy"-based factor. This finding is consistent with viewing mating tactics and mating strategies as distinct and varying independently. An important implication of this study is that measurement of mating tactics is not indicative of underlying mating strategies in women. Further, four patterns of female mating style emerged upon review of participant factor scores and are discussed within an evolutionary context.     

Sexual practices and the duration of last heterosexual encounter: findings from the Australian longitudinal study of health and relationships

This study investigated the association between sexual practices and duration of a sexual encounter. Using data from a population-based computer-assisted telephone survey of 8,656 Australians aged 16 to 64 years, four distinct patterns of sexual practices among respondents were found: "basic sexual encounter" (involving mainly kissing, cuddling, stroking one's partner and being stroked, and vaginal intercourse), "basic sexual encounter plus oral sex," "all assessed sexual practices" (all sexual behaviors included in the survey), and "mainly vaginal intercourse" (characterized by lower levels of kissing, cuddling, and stroking). For both men and women, respondents classified in the basic sexual encounter plus oral sex, and all assessed sexual practices clusters reported significantly longer durations than those in the basic sexual encounter group, whereas respondents in the mainly vaginal intercourse cluster reported shorter durations. These differences were found even after allowing for demographic differences in sexual duration-specifically, age and partner type of the most recent opposite-sex partner. These findings add to the understanding of what typically happens in a sexual encounter. Overall, longer sexual encounters appear to be associated with the inclusion of the least common sexual practices-namely, oral sex and self-stimulation.     

Municipal Law Enforcement Policy on Illegal Immigration Stops: Do Social Factors Determine How Aggressively Local Police Respond to Unauthorized Immigrants?

The devolution of immigration law has increased local police involvement in the enforcement of federal immigration law. But only some departments have responded by implementing policies that restrict their officers' involvement in order to protect residents from overzealous policing. We test theories of economic and ethnic threat to explore the structural characteristics of cities that explain this variation. Multiple regression results show that the most unequal cities that have large Hispanic populations are less likely to have a policy. Hispanic white segregation and high unemployment also reduce the likelihood of policy implementation. We discuss theoretical and policy implications.     

The ideal physician entrepreneur

How does the sometimes elusive and high-stakes world of venture capital really work? How can physician executives with innovative ideas or new technologies approach venture capitalists to help them raise capital to form a start-up company? These important questions are explored in this new column on the physician as entrepreneur. The ideal physician executive is described as: (1) an expert in an area that Wall Street perceives as hot; (2) a public speaker who can enthusiastically communicate scientific and business plans to a variety of audiences; (3) a team leader who is willing to share equity in the company with other employees; (4) a recruiter and a motivator; (5) an implementer who can achieve milestones quickly that allow the company to go public as soon as possible; and (6) a realist who does not resent the terms of the typical deal. The lucrative world of the venture capitalists is foreign territory for physician executives and requires a great idea, charisma, risk-taking, connections, patience, and perseverance to navigate it successfully.     

Simulation for the complex Bingham distribution

The complex Bingham distribution is relevant for the shape analysis of landmark data in two dimensions. In this paper it is shown that the problem of simulating from this distribution reduces to simulation from a truncated multivariate exponential distribution. Several simulation methods are described and their efficiencies are compared.