879 research outputs found
Single Incision Laparoscopic Splenectomy in a 5-Year-Old With Hereditary Spherocytosis
Laparoscopic splenectomy in a pediatric patient was performed through a single umbilical incision by using 3 ports
Estimating a population cumulative incidence under calendar time trends
Abstract
Background
The risk of a disease or psychiatric disorder is frequently measured by the age-specific cumulative incidence. Cumulative incidence estimates are often derived in cohort studies with individuals recruited over calendar time and with the end of follow-up governed by a specific date. It is common practice to apply the Kaplan\u2013Meier or Aalen\u2013Johansen estimator to the total sample and report either the estimated cumulative incidence curve or just a single point on the curve as a description of the disease risk.
Methods
We argue that, whenever the disease or disorder of interest is influenced by calendar time trends, the total sample Kaplan\u2013Meier and Aalen\u2013Johansen estimators do not provide useful estimates of the general risk in the target population. We present some alternatives to this type of analysis.
Results
We show how a proportional hazards model may be used to extrapolate disease risk estimates if proportionality is a reasonable assumption. If not reasonable, we instead advocate that a more useful description of the disease risk lies in the age-specific cumulative incidence curves across strata given by time of entry or perhaps just the end of follow-up estimates across all strata. Finally, we argue that a weighted average of these end of follow-up estimates may be a useful summary measure of the disease risk within the study period.
Conclusions
Time trends in a disease risk will render total sample estimators less useful in observational studies with staggered entry and administrative censoring. An analysis based on proportional hazards or a stratified analysis may be better alternatives
Large-area inventory of species composition using airborne laser scanning and hyperspectral data
5openInternationalInternational coauthor/editorTree species composition is an essential attribute in stand-level forest management inventories and remotely sensed data might be useful for its estimation. Previous studies on this topic have had several operational drawbacks, e.g., performance studied at a small scale and at a single tree-level with large fieldwork costs. The current study presents the results from a large-area inventory providing species composition following an operational area-based approach. The study utilizes a combination of airborne laser scanning and hyperspectral data and 97 field sample plots of 250 m2 collected over 350 km2 of productive forest in Norway. The results show that, with the availability of hyperspectral data, species-specific volume proportions can be provided in operational forest management inventories with acceptable results in 90% of the cases at the plot level. Dominant species were classified with an overall accuracy of 91% and a kappa-value of 0.73. Species-specific volumes were estimated with relative root mean square differences of 34%, 87%, and 102% for Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.), and deciduous species, respectively. A novel tree-based approach for selecting pixels improved the results compared to a traditional approach based on the normalized difference vegetation index.openØrka, Hans Ole; Hansen, Endre Hofstad; Dalponte, Michele; Gobakken, Terje; Næsset, ErikØrka, H.O.; Hansen, E.H.; Dalponte, M.; Gobakken, T.; Næsset, E
Colloidal stabilization via nanoparticle haloing
We present a detailed numerical study of effective interactions between
micron-sized silica spheres, induced by highly charged zirconia nanoparticles.
It is demonstrated that the effective interactions are consistent with a
recently discovered mechanism for colloidal stabilization. In accordance with
the experimental observations, small nanoparticle concentrations induce an
effective repulsion that counteracts the intrinsic van der Waals attraction
between the colloids and thus stabilizes the suspension. At higher nanoparticle
concentrations an attractive potential is recovered, resulting in reentrant
gelation. Monte Carlo simulations of this highly size-asymmetric mixture are
made possible by means of a geometric cluster Monte Carlo algorithm. A
comparison is made to results obtained from the Ornstein-Zernike equations with
the hypernetted-chain closure
A simple, effective, green method for regioselective 3-acylation of unprotected indoles
A fast and green method is developed for regioselective acylation of indoles in the 3-position without the need for protection of the NH position. The method is based on Friedel-Crafts acylation using acid anhydrides. The method has been optimized, and Y(OTf)3 in catalytic amounts is found to be the best catalyst together with the commercially available ionic liquid [BMI]BF4 (1-butyl-3-methylimidazolium tetrafluoro-borate) as solvent. The reaction is completed in a very short time using monomode microwave irradiation. The catalyst can be reused up to four times without significant loss of activity. A range of substituted indoles are investigated as substrates, and thirteen new compounds have been synthesized
Efficient wave function matching approach for quantum transport calculations
The Wave Function Matching (WFM) technique has recently been developed for
the calculation of electronic transport in quantum two-probe systems. In terms
of efficiency it is comparable with the widely used Green's function approach.
The WFM formalism presented so far requires the evaluation of all the
propagating and evanescent bulk modes of the left and right electrodes in order
to obtain the correct coupling between device and electrode regions. In this
paper we will describe a modified WFM approach that allows for the exclusion of
the vast majority of the evanescent modes in all parts of the calculation. This
approach makes it feasible to apply iterative techniques to efficiently
determine the few required bulk modes, which allows for a significant reduction
of the computational expense of the WFM method. We illustrate the efficiency of
the method on a carbon nanotube field-effect-transistor (FET) device displaying
band-to-band tunneling and modeled within the semi-empirical Extended H\"uckel
theory (EHT) framework.Comment: Submitted to Phys. Rev.
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