1,244 research outputs found
lassopack: Model selection and prediction with regularized regression in Stata
This article introduces lassopack, a suite of programs for regularized
regression in Stata. lassopack implements lasso, square-root lasso, elastic
net, ridge regression, adaptive lasso and post-estimation OLS. The methods are
suitable for the high-dimensional setting where the number of predictors
may be large and possibly greater than the number of observations, . We
offer three different approaches for selecting the penalization (`tuning')
parameters: information criteria (implemented in lasso2), -fold
cross-validation and -step ahead rolling cross-validation for cross-section,
panel and time-series data (cvlasso), and theory-driven (`rigorous')
penalization for the lasso and square-root lasso for cross-section and panel
data (rlasso). We discuss the theoretical framework and practical
considerations for each approach. We also present Monte Carlo results to
compare the performance of the penalization approaches.Comment: 52 pages, 6 figures, 6 tables; submitted to Stata Journal; for more
information see https://statalasso.github.io
Optimal Scaling transformations to model non-linear relations in GLMs with ordered and unordered predictors
In Generalized Linear Models (GLMs) it is assumed that there is a linear
effect of the predictor variables on the outcome. However, this assumption is
often too strict, because in many applications predictors have a nonlinear
relation with the outcome. Optimal Scaling (OS) transformations combined with
GLMs can deal with this type of relations. Transformations of the predictors
have been integrated in GLMs before, e.g. in Generalized Additive Models.
However, the OS methodology has several benefits. For example, the levels of
categorical predictors are quantified directly, such that they can be included
in the model without defining dummy variables. This approach enhances the
interpretation and visualization of the effect of different levels on the
outcome. Furthermore, monotonicity restrictions can be applied to the OS
transformations such that the original ordering of the category values is
preserved. This improves the interpretation of the effect and may prevent
overfitting. The scaling level can be chosen for each individual predictor such
that models can include mixed scaling levels. In this way, a suitable
transformation can be found for each predictor in the model. The implementation
of OS in logistic regression is demonstrated using three datasets that contain
a binary outcome variable and a set of categorical and/or continuous predictor
variables.Comment: 35 pages, 4 figure
Collapse from the top: Brushes of poly(N-isopropylacrylamide) in co-nonsolvent mixtures
Using a combination of ellipsometry and friction force microscopy, we study the reversible swelling, collapse and variation in friction properties of covalently bound poly(N-isopropylacrylamide) (PNIPAM) layers on silicon with different grafting densities in response to exposure to good solvents and cononsolvent mixtures. Changes in the thickness and segment density distribution of grafted films are investigated by in situ ellipsometry. Based on quantitative modelling of the ellipsometry spectra, we postulate a structural model, which assumes that collapse takes place in the contacting layer between the brush and the co-nonsolvent and the top-collapsed brushes remain hydrated in the film interior. Using the structural model derived from ellipsometry spectra, we analyse the AFM based friction force microscopy data, which were obtained by silica colloidal probes. Results show a large increase of the friction coefficient of PNIPAM grafts when the grafts swollen by water are brought in contact with cononsolvents. For instance, the value of the friction coefficient for a medium density brush in water is four times lower than the value observed in a waterāmethanol (50% v/v) mixture. This increase of friction is
accompanied by an increase in adherence between the PNIPAM chains and the silica colloidal probes, and is a result of chain collapse in the graft when contacted by a co-nonsolvent mixture in agreement with the model postulated on the basis of ellipsometric characterisation. The kinetic behaviour of the collapse is assessed by measuring the temporal variation of friction in situ as a function of elapsed time
following contact with the co-nonsolvent as a function of graft density. In conclusion, the effect of cononsolvency influenced both the thickness of the PNIPAM brushes and the tribological behavior of the brush surfaces
Functional improvement and correlations with symptomatic improvement in adults with attention deficit hyperactivity disorder receiving long-acting methylphenidate
Item does not contain fulltextBACKGROUND: Data on the relationship between core symptoms and daily functioning in adults with attention deficit hyperactivity disorder (ADHD) are limited. Daily functioning was assessed as part of an open-label extension, and associations with symptom scores were evaluated. METHOD: After a 5-week double-blind study with adults with ADHD receiving osmotic-controlled release oral delivery system (OROS) methylphenidate (MPH) 18, 36 or 72 mg/day, or placebo, participants were eligible for a 7-week open-label extension in which all patients received OROS MPH. Data for the Conners' Adult ADHD Rating Scale - Observer: Screening Version (CAARS-O:SV) (primary endpoint) have been presented previously. Secondary endpoints included the observer self-reported short version of the CAARS (CAARS-S:S) and the Clinical Global Impressions - Severity Scale (CGI-S). Daily functioning and quality of life were assessed using the Sheehan Disability Scale (SDS) and the Quality of Life Enjoyment and Satisfaction Questionnaire (Q-LES-Q) respectively. In post-hoc analyses, changes in CAARS-O:SV were evaluated in subgroups. Relationships between symptom and functional outcomes were evaluated in a multivariate regression analysis. RESULTS: A total of 370 patients entered the open-label extension. Significant improvements from baseline in CAARS-O:SV were similar regardless of sex, ADHD subtype, prior treatment or psychiatric co-morbidity. Significant improvements from double-blind baseline were also seen for the CAARS-S:S, CGI-S, SDS and Q-LES-Q. Improvements in the CAARS-O:SV Hyperactivity/Impulsivity subscale were associated with improvements in SDS total and subscale scores, and in the Q-LES-Q score at open-label endpoint. Improvements in CAARS-O:SV Inattention subscale and CGI-S scores were not significantly associated with functional changes. CONCLUSIONS: Improvements in ADHD symptoms relating to hyperactivity and impulsivity in adults receiving OROS MPH are associated with improvements in daily functioning and quality of life.1 januari 201
Droplet impact on hydrophobic surfaces with hierarchical roughness
We investigate the dynamic properties of microliter droplets impacting with velocities up to on hydrophobic surfaces with hierarchical roughness. The substrates consist of multiple layers of silica microspheres, which are decorated with gold nanoparticles; the superstructures are hydrophobized by chemical modification. The initial impact event is analysed, primarily focusing on the bouncing of the droplets. The number of bounces increases exponentially with substrate hydrophobicity as expressed by the contact angle. The subsequent relaxation regime is analysed in terms of the frequency and damping rate of the droplet oscillations. Both quantities exhibit a substantial decrease for large contact angles. Results are discussed in relation to reports in literature; damping is most likely due to viscous dissipation
Vortices in vibrated granular rods
We report the experimental observation of novel vortex patterns in vertically
vibrated granular rods. Above a critical packing fraction, moving ordered
domains of nearly vertical rods spontaneously form and coexist with horizontal
rods. The domains of vertical rods coarsen in time to form large vortices. We
investigate the conditions under which the vortices occur by varying the number
of rods, vibration amplitude and frequency. The size of the vortices increases
with the number of rods. We characterize the growth of the ordered domains by
measuring the area fraction of the ordered regions as a function of time. A
{\em void filling} model is presented to describe the nucleation and growth of
the vertical domains. We track the ends of the vertical rods and obtain the
velocity fields of the vortices. The rotation speed of the rods is observed to
depend on the vibration velocity of the container and on the packing. To
investigate the impact of the direction of driving on the observed phenomena,
we performed experiments with the container vibrated horizontally. Although
vertical domains form, vortices are not observed. We therefore argue that the
motion is generated due to the interaction of the inclination of the rods with
the bottom of a vertically vibrated container. We also perform simple
experiments with a single row of rods in an annulus. These experiments directly
demonstrate that the rod motion is generated when the rods are inclined from
the vertical, and is always in the direction of the inclination.Comment: 6 pages, 10 figure, 2 movies at http://physics.clarku.edu/vortex uses
revtex
Mathematics in different settings: plenary panel.
When we think about the title āMathematics in different settingsā, a number of questions arise. For example:
ā¢ How many mathematics are there ā one or many? Is there a mathematics that is āprior toā, or independent of, any setting?
ā¢ What (who) is it that makes settings ādifferentā? And how does this relate to social differences among people?
ā¢ What is an appropriate typology of different settings ā for research or for curriculum design purposes? Relatedly, we might ask: who decides what is āimportantā?
ā¢ What is the nature of relations among policy arrangements, research and educational institutional settings?
ā¢ How are different settings represented in mathematics teaching and assessment?
ā¢ What is the relationship of mathematics education researchers to any setting
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