Asking Only “Did You Use a Condom?” Underestimates the Prevalence of Unprotected Sex Among Perinatally HIV Infected and Perinatally Exposed But Uninfected Youth

Among young adults who use condoms, incomplete condom use (putting a condom on after beginning or taking a condom off before finishing sex) and condom failure (condom breaking or slipping off during sex) are common. Therefore, sexual behavior surveys that ask only if a condom was used are likely to underestimate the actual prevalence of unprotected sex. This study examined data from 135 sexually active perinatally HIV-infected (PHIV+) youth and perinatally exposed but uninfected (PHIV?) youth, ages 13 to 24. Participants were asked whether they used a condom on their first and their most recent occasion of vaginal sex. Youth who reported using a condom were asked a follow-up question about whether there was any time during that occasion when sex was not protected by a condom. This follow-up question identified additional participants— almost double the proportions who initially said they did not use a condom—who had unprotected sex. Incomplete condom use was similar among PHIV+ and PHIV?youth, boys and girls, Latinos and African Americans, and younger and older youth. These findings illustrate the importance of asking specifically about whether any unprotected behavior occurred from start to finish of sex to achieve more valid estimates of sexual risk behavior.     

Cross-infection in epidemics spread by carriers

A block-structured transient Markov process is introduced to describe an epidemic spreading within two linked populations, of carriers and susceptibles. The epidemic terminates as soon as there are no more carriers or susceptibles present in the population. Our purpose is to determine the distribution of the final susceptible and carrier states, and of any integral path for the susceptible process. The transient epidemic state is also briefly discussed. Then, the model is extended to allow the recovery of infected individuals. Finally, several particular models, some known, are used for illustration.     

The generalized symmetric eigenproblem in multivariate statistical models

The solution of the generalized symmetric eigenproblem Ax = λBx is required in many multivariate statistical models, viz. canonical correlation, discriminant analysis, multivariate linear model, limited information maximum likelihoods. The problem can be solved by two efficient numerical algorithms: Cholesky decomposition or singular value decomposition. Practical considerations for implementation are also discussed.     

Effects of Uncertainties on Exposure Estimates to Methylmercury: A Monte Carlo Analysis of Exposure Biomarkers versus Dietary Recall Estimation

This article presents a general model for estimating population heterogeneity and "lack of knowledge" uncertainty in methylmercury (MeHg) exposure assessments using two-dimensional Monte Carlo analysis. Using data from fish-consuming populations in Bangladesh, Brazil, Sweden, and the United Kingdom, predictive model estimates of dietary MeHg exposures were compared against those derived from biomarkers (i.e., [Hg]hair and [Hg]blood). By disaggregating parameter uncertainty into components (i.e., population heterogeneity, measurement error, recall error, and sampling error) estimates were obtained of the contribution of each component to the overall uncertainty. Steady-state diet:hair and diet:blood MeHg exposure ratios were estimated for each population and were used to develop distributions useful for conducting biomarker-based probabilistic assessments of MeHg exposure. The 5th and 95th percentile modeled MeHg exposure estimates around mean population exposure from each of the four study populations are presented to demonstrate lack of knowledge uncertainty about a best estimate for a true mean. Results from a U.K. study population showed that a predictive dietary model resulted in a 74% lower lack of knowledge uncertainty around a central mean estimate relative to a hair biomarker model, and also in a 31% lower lack of knowledge uncertainty around central mean estimate relative to a blood biomarker model. Similar results were obtained for the Brazil and Bangladesh populations. Such analyses, used here to evaluate alternative models of dietary MeHg exposure, can be used to refine exposure instruments, improve information used in site management and remediation decision making, and identify sources of uncertainty in risk estimates.     

Improved Self-exclusion Program: Preliminary Results

The gambling industry has offered self-exclusion programs for quite a long time. Such measures are designed to limit access to gaming opportunities and provide problem gamblers with the help they need to cease or limit their gambling behaviour. However, few studies have empirically evaluated these programs. This study has three objectives: (1) to observe the participation in an improved self-exclusion program that includes an initial voluntary evaluation, phone support, and a mandatory meeting, (2) to evaluate satisfaction and usefulness of this service as perceived by self-excluders, (3) to measure the preliminary impact of this improved program. One hundred sixteen self-excluders completed a questionnaire about their satisfaction and their perception of the usefulness during the mandatory meeting. Among those participants, 39 attended an initial meeting. Comparisons between data collected at the initial meeting and data taken at the final meeting were made for those 39 participants. Data showed that gamblers chose the improved self-exclusion program 75% of the time; 25% preferred to sign a regular self-exclusion contract. Among those who chose the improved service, 40% wanted an initial voluntary evaluation and 37% of these individuals actually attended that meeting. Seventy percent of gamblers came to the mandatory meeting, which was a required condition to end their self-exclusion. The majority of participants were satisfied with the improved self-exclusion service and perceived it as useful. Major improvements were observed between the final and the initial evaluation on time and money spent, consequences of gambling, DSM-IV score, and psychological distress. The applicability of an improved self-exclusion program is discussed and, as shown in our study, the inclusion of a final mandatory meeting might not be so repulsive for self-excluders. Future research directives are also proposed.

Impact of Visiting an Onsite Casino Information Centre on Perceptions about Randomness and Gambling Behaviours

At the beginning of 2000, some educational initiatives in the field of responsible gambling resulted in the implementation of Onsite Casino Information Centres (OCICs). However, no study has yet empirically evaluated the impact of visiting an OCIC. This paper includes two studies evaluating the OCIC Au Centre du Hasard, located in Montreal, Quebec. The goal of the first study was to identify the profile of the visitors and to assess their appreciation. After a visit, 336 patrons accepted to complete a pen and paper questionnaire. The goal of the second study was to evaluate the impact of a visit on the perceptions about randomness and the gambling behaviours of the visitors. For this study, 67 visitors were evaluated before, after, and 3 months following a visit and their results were compared to a control group. Data showed that most visitors were seniors, occasional slot machine gamblers, and in control of their gambling activities. The majority of guests greatly appreciated their visit. A visit to Au Centre du Hasard seemed to modify the misconceptions towards the notion of randomness but not the gambling behaviours. These gains were maintained at 3-month follow-up. Results with respect to other prevention programs are discussed, and future research avenues are suggested.

Least squares adaptive polynomials

A concept of adaptive least squares polynomials is introduced for modelling time series data. A recursion algorithm for updating coefficients of the adaptive polynomial (of a fixed degree) is derived. This concept assumes that the weights w are such that i) the importance of the data values, in terms of their weights, relative to each other stays fixed, and that ii) they satisfy the update property, i.e., the polynomial does not change if a new data value is a polynomial extrapolate. Closed form results are provided for exponential weights as a special case as they are shown to possess the update property when used with polynomials.

The concept of adaptive polynomials is similar to the linear adaptive prediction provided by the Kalman filter or the Least Mean Square algorithm of Widrow and Hoff. They can be useful in interpolating, tracking and analyzing nonstationary data.