How useful are the stages of change for targeting interventions? randomized test of a brief intervention to reduce smoking

Objectives. To see whether the stages of change are useful for targeting a brief intervention to reduce smoking based on implementation intentions. A second objective was to rule out demand characteristics as an alternative explanation for the findings of intervention studies based on the transtheoretical model and implementation intentions. Design. Participants (N = 350) were randomized to a passive control condition (questionnaire only), active control condition (questionnaire-plus-instruction to plan to quit) or experimental condition (questionnaire, plan to quit, form an implementation intention). Their behavior and psychosocial orientation to quit were measured at baseline and 2-month follow-up. Main Outcome Measures. Theory of planned behavior variables, nicotine dependence and quitting. Results. Significantly more people quit smoking in the experimental condition than in the control conditions, and the planning instructions changed intention to quit and perceived control over quitting, but not behavior. Stage of change moderated these effects such that implementation intentions worked best for individuals who were in the preparation stage at baseline. Conclusion. Harnessing both motivational and volitional processes seems to enhance the effectiveness of smoking cessation programs, although further work is required to clarify inconsistencies in the literature using the stages of change.</p

Accretion Disc Evolution in Single and Binary T Tauri Stars

We present theoretical models for the evolution of T Tauri stars surrounded by circumstellar discs. The models include the effects of pre-main-sequence stellar and time dependent disc evolution, and incorporate the effects of stellar magnetic fields acting on the inner disc. For single stars, consistency with observations in Taurus-Auriga demands that disc dispersal occurs rapidly, on much less than the viscous timescale of the disc, at roughly the epoch when heating by stellar radiation first dominates over internal viscous dissipation. Applying the models to close binaries, we find that because the initial conditions for discs in binaries are uncertain, studies of extreme mass ratio systems are required to provide a stringent test of theoretical disc evolution models. We also note that no correlation of the infra-red colours of T Tauri stars with their rotation rate is observed, in apparent contradiction to the predictions of simple magnetospheric accretion models.Comment: 11 pages, MNRAS in pres

Massive planet migration: Theoretical predictions and comparison with observations

We quantify the utility of large radial velocity surveys for constraining theoretical models of Type II migration and protoplanetary disk physics. We describe a theoretical model for the expected radial distribution of extrasolar planets that combines an analytic description of migration with an empirically calibrated disk model. The disk model includes viscous evolution and mass loss via photoevaporation. Comparing the predicted distribution to a uniformly selected subsample of planets from the Lick / Keck / AAT planet search programs, we find that a simple model in which planets form in the outer disk at a uniform rate, migrate inward according to a standard Type II prescription, and become stranded when the gas disk is dispersed, is consistent with the radial distribution of planets for orbital radii 0.1 AU < a < 2.5 AU and planet masses greater than 1.65 Jupiter masses. Some variant models are disfavored by existing data, but the significance is limited (~95%) due to the small sample of planets suitable for statistical analysis. We show that the favored model predicts that the planetary mass function should be almost independent of orbital radius at distances where migration dominates the massive planet population. We also study how the radial distribution of planets depends upon the adopted disk model. We find that the distribution can constrain not only changes in the power-law index of the disk viscosity, but also sharp jumps in the efficiency of angular momentum transport that might occur at small radii.Comment: ApJ, in press. References updated to match published versio

Predicting and explaining transtheoretical model stage transitions in relation to condom-carrying behaviour

Objectives: The stages of change component of the transtheoretical model has been applied to safe sex behaviours in cross-sectional analyses, but have not yet been subject to prospective analysis. It was predicted that: (a) cross-sectional analyses would demonstrate good discrimination between the stages of change, (b) prospective analyses would allow for the identification of predictors of stage transitions, and (c) implementation intentions would explain progression from the preparation stage. Design: This study employed an experimental longitudinal design. Participants were randomly assigned to the experimental (implementation intention) or control conditions and completed questionnaires at baseline and at 2-months follow up. Methods: 525 adolescents who were broadly representative of the UK population completed questionnaires at baseline and follow up (n = 393) measuring: demographic variables, stage of change, theory of planned behaviour constructs, anticipated regret and moral norm in relation to condom carrying behaviour. The experimental condition completed a self-generated implementation intention to carry condoms at the end of the baseline questionnaire. Results: Discriminant function analyses indicated that the stages of change could be accurately discriminated from one another cross-sectionally and that, longitudinally, the measured variables were able to predict transitions between most stages. Implementation intentions caused people to progress from the preparation stage. Conclusions: Transitions between most stages were reliably predicted thereby providing potential targets for intervention. The brief implementation intention intervention was effective and could easily be utilised and expanded to encompass a broader range of sexual health behaviours.</p

MHD simulations of the collapsar model for GRBs

We present results from axisymmetric, time-dependent magnetohydrodynamic (MHD) simulations of the collapsar model for gamma-ray bursts. Our main conclusion is that, within the collapsar model, MHD effects alone are able to launch, accelerate and sustain a strong polar outflow. We also find that the outflow is Poynting flux-dominated, and note that this provides favorable initial conditions for the subsequent production of a baryon-poor fireball.Comment: 4 pages, to appear in proceedings of "2003 GRB Conference" (Santa Fe, NM, September 8-12, 2003), needs aipprocs LaTeX class, movies are available at http://rocinante.colorado.edu/~proga

Resonant state expansion applied to planar waveguides

The resonant state expansion, a recently developed method in electrodynamics, is generalized here to planar open optical systems with non-normal incidence of light. The method is illustrated and verified on exactly solvable examples, such as a dielectric slab and a Bragg reflector microcavity, for which explicit analytic formulas are developed. This comparison demonstrates the accuracy and convergence of the method. Interestingly, the spectral analysis of a dielectric slab in terms of resonant states reveals an influence of waveguide modes in the transmission. These modes, which on resonance do not couple to external light, surprisingly do couple to external light for off-resonant excitation

Reduction of Effective Terahertz Focal Spot Size By Means Of Nested Concentric Parabolic Reflectors

An ongoing limitation of terahertz spectroscopy is that the technique is generally limited to the study of relatively large samples of order 4 mm across due to the generally large size of the focal beam spot. We present a nested concentric parabolic reflector design which can reduce the terahertz focal spot size. This parabolic reflector design takes advantage of the feature that reflected rays experience a relative time delay which is the same for all paths. The increase in effective optical path for reflected light is equivalent to the aperture diameter itself. We have shown that the light throughput of an aperture of 2 mm can be increased by a factor 15 as compared to a regular aperture of the same size at low frequencies. This technique can potentially be used to reduce the focal spot size in terahertz spectroscopy and enable the study of smaller samples

Lifetime cost effectiveness of simvastatin in a range of risk groups and age groups derived from a randomised trial of 20,536 people

&lt;i&gt;Objectives&lt;/i&gt;: To evaluate the cost effectiveness of 40 mg simvastatin daily continued for life in people of different ages with differing risks of vascular disease. Design A model developed from a randomised trial was used to estimate lifetime risks of vascular events and costs of treatment and hospital admissions in the United Kingdom. &lt;i&gt;Setting&lt;/i&gt;: 69 hospitals in the UK. &lt;i&gt;Participants&lt;/i&gt;: 20 536 men and women (aged 40-80) with coronary disease, other occlusive arterial disease, or diabetes. &lt;i&gt;Interventions&lt;/i&gt;: 40 mg simvastatin daily versus placebo for an average of 5 years. &lt;i&gt;Main&lt;/i&gt; &lt;i&gt;outcome&lt;/i&gt; &lt;i&gt;measures&lt;/i&gt;: Cost effectiveness of 40 mg simvastatin daily expressed as additional cost per life year gained. Major vascular event defined as non-fatal myocardial infarction or death from coronary disease, any stroke, or revascularisation procedure. Results were extrapolated to younger and older age groups at lower risk of vascular disease than were studied directly, as well as to lifetime treatment. &lt;i&gt;Results&lt;/i&gt;: At the April 2005 UK price of £4.87 (&#8364;7; $9) per 28 day pack of generic 40 mg simvastatin, lifetime treatment was cost saving in most age groups and vascular disease risk groups studied directly. Gains in life expectancy and cost savings decreased with increasing age and with decreasing risk of vascular disease. People aged 40-49 with 5 year risks of major vascular events of 42% and 12% at start of treatment gained 2.49 and 1.67 life years, respectively. Treatment with statins remained cost saving or cost less than £2500 per life year gained in people as young as 35 years or as old as 85 with 5 year risks of a major vascular event as low as 5% at the start of treatment. &lt;i&gt;Conclusions&lt;/i&gt;: Treatment with statins is cost effective in a wider population than is routinely treated at present

Travelling waves in a nonlinear degenerate diffusion model for bacterial pattern formation

We study a reaction diffusion model recently proposed in [5] to describe the spatiotemporal evolution of the bacterium Bacillus subtilis on agar plates containing nutrient. An interesting mathematical feature of the model, which is a coupled pair of partial differential equations, is that the bacterial density satisfies a degenerate nonlinear diffusion equation. It was shown numerically that this model can exhibit quasi-one-dimensional constant speed travelling wave solutions. We present an analytic study of the existence and uniqueness problem for constant speed travelling wave solutions. We find that such solutions exist only for speeds greater than some threshold speed giving minimum speed waves which have a sharp profile. For speeds greater than this minimum speed the waves are smooth. We also characterise the dependence of the wave profile on the decay of the front of the initial perturbation in bacterial density. An investigation of the partial differential equation problem establishes,via a global existence and uniqueness argument, that these waves are the only long time solutions supported by the problem. Numerical solutions of the partial differential equation problem are presented and they confirm the results of the analysis