| 741. |
- Johansson, Sandra, et al.
(författare)
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Generation of turquoise light by sum-frequency mixing of a diode-pumped solid-state laser and a laser diode in periodically poled PPKTP
- 2004
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Ingår i: Optics expresss. - 1094-4087. ; 12, s. 4935-4940
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Tidskriftsartikel (refereegranskat)abstract
- We report a simple and efficient method to achieve visible light by sum-frequency mixing radiation from a diode-pumped solid-state laser and a laser diode in a periodically poled KTiOPO4 crystal. Since high-power laser diodes are available at a wide range of wavelengths, it is thereby possible to obtain essentially any wavelength in the visible spectrum by appropriate choice of lasers. For demonstration we choose to construct a light source in the blue-green region. A turquoise output power of 4.0 mW was achieved.
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| 742. |
- Johansson, Stefan, 1980, et al.
(författare)
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Globalised student achievement? A longitudinal and cross-country analysis of convergence in mathematics performance
- 2019
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Ingår i: Comparative Education. - : Informa UK Limited. - 0305-0068 .- 1360-0486. ; 55:4, s. 536-556
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Tidskriftsartikel (refereegranskat)abstract
- With the aid of longitudinal country-level data from five IEA TIMSS assessments (1995–2011), the current study addresses the issue of the globalisation of curricula and achievement. To explore the hypothesis of global convergence, we study performance in four subdomains of mathematics. Using regression with fixed effects for countries, we consider whether the variation of subdomain scores decreases globally over time. Additionally, we explore qualitative differences in performance profiles using latent class analysis. Our results provide little evidence for a global harmonisation of student achievement. Rather, for regions with a similar language and culture, we observe similar strengths and weaknesses in mathematics content areas. Furthermore, these patterns remain stable over time. Directions for future research include the exploration of global trends in aspects of attained curricula for other subjects, and the use of information on school achievement.
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| 743. |
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| 744. |
- Johansson, Stefan
(författare)
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High order difference approximations for the linearized Euler equations
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The computers of today make it possible to do direct simulation of aeroacoustics, which is very computational demanding since a very high resolution is needed.In the present thesis we study issues of relevance for aeroacoustic simulations. Paper A considers standard high order difference methods. We study two different ways to apply boundary conditions in a stable way. Numerical experiments are done for the 1D linearized Euler equations.In paper B we develop difference methods which give smaller dispersion errors than standard central difference methods. The new methods are applied to the 1D wave equation.Finally in Paper C we apply the new difference methods to aeroacoustic simulations based on the 2D linearized Euler equations.Taken together, the methods presented here lead to better approximation of the wave number, which in turn results in a smaller L2-error than obtained by previous methods found in the literature. The results are valid when the problem is not fully resolved, which usually is the case for large scale applications.
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| 745. |
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| 746. |
- Johansson, Stefan
(författare)
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High order summation by parts operator based on a DRP scheme applied to 2D Aeroacoustics
- 2004
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A strictly stable high order finite difference method based on Tam and Webb's dispersion relation preserving scheme in the interior has been verified for a 2D aeroacoustic problem. Results show that the method gives lower dispersion error than a similar method derived by Strand, which is based on standard sixth order difference approximation in the interior, when boundary effects are not important.
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| 747. |
- Johansson, Stefan, et al.
(författare)
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Implementation of an OFDM Synchronization Algorithm
- 1999
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Ingår i: 42nd Midwest Symposium on Circuits and Systems, 1999.. - 0780354915 ; 1, s. 228-231
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Konferensbidrag (refereegranskat)abstract
- In this paper, a implementation of an OFDM synchronization algorithm is presented. Both time and frequency offsets are estimated simultaneously, using the cyclic prefix. The algorithm is implemented on a commercial DSP, an FPGA and as an ASIC. It is shown that although the algorithm is too complex to be implemented on a standard DSP, a hardware architecture that is optimized for the algorithm can be implemented with moderate complexity
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| 748. |
- Johansson, Stefan, et al.
(författare)
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Infection with Parvovirus B19 and Herpes viruses in early pregnancy and risk of second trimester miscarriage or very preterm birth
- 2008
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Ingår i: Reproductive Toxicology. - : Elsevier BV. - 0890-6238 .- 1873-1708. ; 26:3-4, s. 298-302
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Tidskriftsartikel (refereegranskat)abstract
- We investigated whether infections with Parvovirus B19 and Herpes viruses in early pregnancy increase risks of second trimester miscarriage or delivery before 32 gestational weeks. Blood samples taken in early pregnancy were analyzed for Parvovirus B19 or Herpes viruses. Viremia was found in blood samples of 11 (4.7%) women with second trimester miscarriage and 10 (3.7%) women with very preterm birth, compared to 5 (1.7%) women who delivered at term, corresponding to adjusted odds ratios [95% CI] of 3.32 [0.93, 11.8] and 2.21 [0.71, 6.84], respectively. In stratified analyses, Parvovirus B19 viremia was associated with adjusted odds ratios of 3.76 [0.77, 18.3] for second trimester miscarriage and 2.66 [0.64, 11.1] for very preterm birth. Corresponding odds ratios for Human Herpes virus 6 viremia was 2.52 [0.33, 19.5] and 1.08 [0.14, 8.08], respectively. In conclusion, this study lends some support to the hypothesis that women with viremia in early pregnancy may face an increased risk of second trimester miscarriage or very preterm birth. Studies with larger sample sizes are needed.
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| 749. |
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| 750. |
- Johansson, Stefan, 1980, et al.
(författare)
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Is there a trend towards convergence in countries mathematics achievement?
- 2015
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Ingår i: Comparative and International Education Society | 59th Annual Conference “Ubuntu! Imagining a Humanist Education Globally” March 8 –13, 2015, Washington, D.C..
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Purposes The main purpose of this paper is to explore if there has been a harmonization of countries’ performances, resulting in a “world curriculum”. If countries tend to converge with respect to their response patterns to mathematics tests, this may be an indication of harmonization of their school systems and curricula. For example, if students in different countries show more similar response patterns over time this may be taken as an evidence for a global force, which, in literature has been related to, among others, the international studies. Theoretical framework The international comparative studies have been widely debated in media the last decade. Although these studies have provided valuable information, the studies have met a fair amount of criticism, from different angles and perspectives. One line of this criticism mainly concern the policy borrowing across countries that the international studies have been argued to contribute to: countries curricula tend to converge over time into a so called ‘world curriculum’ (Baker & LeTendre, 2005). Poor performing countries may focus to specific knowledge and skills to achieve high results in the international comparisons. This might hamper other competences such as innovativeness and creativity. Such competences have been argued to decline with increasing results in international comparisons (e.g., Zhao, 2012). However, the empirical evidence in this area is quite limited. Rutkowski and Rutkowski (2009) studied country-level item responses for a range of countries participating in TIMSS 95, 99, and 03. Although they identify similarities on regional level (e.g. East European), their find not global forces. However, the time span this study covers might be too limited to capture global trend that indicate convergence. Analytical methods We use latent profile analysis on aggregated country level data to identify different achievement profiles in mathematics. Countries are included in the dataset multiple times if they participated in different mathematics studies. For each country, achievement information on four content domain subscales is available. The latent profile analysis aims to identify a set of countries (latent classes) that are defined in terms of their score profile on the four content domain subscales. In a second step, we introduce time as an explanatory variable to test the hypothesis of convergence, that is, if fewer latent classes represent more countries. In other words, we investigate if countries become more similar over time. Data sources The current study employs data from several previous IEA studies on mathematics in secondary school: SIMS 1980, and TIMSS 1995, 1999, 2003, 2007, and 2011. More than 80 educational entities (countries or regions) participated in at least one study, and the assessment material covers 744 test items. We used the mathematics assessment framework from the latest TIMSS cycles to assign each test item to one of the four content areas Numbers, Algebra, Geometry, and Data & Chance. This step is necessary because the older studies used different labels (e.g., Arithmetic instead of Numbers). In a second step, we calculated proportion correct scores for four subscales. These scores serve as a basis for the latent profile analysis. To avoid that the latent classes are rather based on the overall performance on the subscales than on qualitative differences in the patterns, we standardized the raw proportion correct statistic within each country and this was done separately for each study. We used a set of dummies for each study as a measure of time. Results and Conclusions Preliminary findings show that it is possible to identify different latent clusters of countries with similar patterns in the four mathematic subdomains. The classes differ with respect to relatively high achievement on certain subdomains of mathematic and in terms of their size, i.e. the number of countries that are best described by the respective clusters’ pattern. However, introducing time as a covariate does not predict class membership. In other words, we did not observe that certain latent classes increase consistently over time. The international studies may have large impact on the school debate and the policy discussions in different countries. The may also constitute a base for decisions and reforms. However, that unintended consequences such as convergence of curricula actually would lead to convergence of actual achievement is more difficult to prove. Even though countries strive to be equally good as the best performing, there is no evidence in this study that they conform in their response patterns over time. Educational Significance The study concerns an issue of great educational importance, which previously has been difficult to study empirically. Moreover, the methodological approach used is also of significant interest.
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