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71.
Simulation‐based evaluation of the linear‐mixed model in the presence of an increasing proportion of singletons 下载免费PDF全文
Robin Bruyndonckx Niel Hens Marc Aerts 《Biometrical journal. Biometrische Zeitschrift》2018,60(1):49-65
Data in medical sciences often have a hierarchical structure with lower level units (e.g. children) nested in higher level units (e.g. departments). Several specific but frequently studied settings, mainly in longitudinal and family research, involve a large number of units that tend to be quite small, with units containing only one element referred to as singletons. Regardless of sparseness, hierarchical data should be analyzed with appropriate methodology such as, for example linear‐mixed models. Using a simulation study, based on the structure of a data example on Ceftriaxone consumption in hospitalized children, we assess the impact of an increasing proportion of singletons (0–95%), in data with a low, medium, or high intracluster correlation, on the stability of linear‐mixed models parameter estimates, confidence interval coverage and F test performance. Some techniques that are frequently used in the presence of singletons include ignoring clustering, dropping the singletons from the analysis and grouping the singletons into an artificial unit. We show that both the fixed and random effects estimates and their standard errors are stable in the presence of an increasing proportion of singletons. We demonstrate that ignoring clustering and dropping singletons should be avoided as they come with biased standard error estimates. Grouping the singletons into an artificial unit might be considered, although the linear‐mixed model performs better even when the proportion of singletons is high. We conclude that the linear‐mixed model is stable in the presence of singletons when both lower‐ and higher level sample sizes are fixed. In this setting, the use of remedial measures, such as ignoring clustering and grouping or removing singletons, should be dissuaded. 相似文献
72.
Kathiravan Meeran Niel Verbrigghe Johannes Ingrisch Lucia Fuchslueger Lena Müller Páll Sigurðsson Bjarni D. Sigurdsson Herbert Wachter Margarete Watzka Jennifer L. Soong Sara Vicca Ivan A. Janssens Michael Bahn 《Global Change Biology》2023,29(18):5276-5291
Climate warming has been suggested to impact high latitude grasslands severely, potentially causing considerable carbon (C) losses from soil. Warming can also stimulate nitrogen (N) turnover, but it is largely unclear whether and how altered N availability impacts belowground C dynamics. Even less is known about the individual and interactive effects of warming and N availability on the fate of recently photosynthesized C in soil. On a 10-year geothermal warming gradient in Iceland, we studied the effects of soil warming and N addition on CO2 fluxes and the fate of recently photosynthesized C through CO2 flux measurements and a 13CO2 pulse-labeling experiment. Under warming, ecosystem respiration exceeded maximum gross primary productivity, causing increased net CO2 emissions. N addition treatments revealed that, surprisingly, the plants in the warmed soil were N limited, which constrained primary productivity and decreased recently assimilated C in shoots and roots. In soil, microbes were increasingly C limited under warming and increased microbial uptake of recent C. Soil respiration was increased by warming and was fueled by increased belowground inputs and turnover of recently photosynthesized C. Our findings suggest that a decade of warming seemed to have induced a N limitation in plants and a C limitation by soil microbes. This caused a decrease in net ecosystem CO2 uptake and accelerated the respiratory release of photosynthesized C, which decreased the C sequestration potential of the grassland. Our study highlights the importance of belowground C allocation and C-N interactions in the C dynamics of subarctic ecosystems in a warmer world. 相似文献
73.
Restriction maps of human adenovirus types 2, 5, and 3 for BstEII, MluI, NdeI, NruI and SfiI endonucleases 总被引:2,自引:0,他引:2
The positions of cleavage sites for BstEII, MluI, NdeI, NruI and SfiI restriction endonucleases in the DNA from human adenovirus (Ad) serotypes 2, 5 and 3 were determined. In addition, the sites of cleavage for BglII in Ad3 DNA were located. All these enzymes possess a narrow specificity and generated a small number of discrete DNA fragments. Ad3 DNA was not cleaved by MluI and SfiI. It was the first observation of the absence of cleavage of an adenovirus DNA by a restriction endonuclease. 相似文献
74.
A Muller E Rechencq C Kugel J P Lellouche J P Beaucourt G Niel J P Girard J C Rossi C Bonne 《Prostaglandins》1989,38(6):635-644
(5,6)-dihydroxy-7,9-trans-11,14-cis-eicosatetraenoic acids [5,6)-DiHETEs) were synthesized and separated into four pure diastereoisomers. They were tested for comparative binding affinities to leukotriene receptors (LTC4, LTD4, LTB4) in guinea pig lung membranes. Only (5S,6R)-DiHETE was recognized by the LTD4 receptor, the other receptors interacted with neither of the four isomers. (5S,6R)-DiHETE also contracted ileum in vitro and this effect was inhibited by the LTD4 receptor antagonists ICI 198,615 and SKF104,353. These data suggest that the bioproduct (5S,6R)-DiHETE generated by enzymatic conversion of LTA4 could have some LTD4-like activity when produced in large concentrations. 相似文献
75.
The use of a metabolically structured model in the study of growth, nitrification, and denitrification by Thiosphaera pantotropha 总被引:6,自引:0,他引:6
Geraats SG Hooijmans CM van Niel EW Robertson LA Heijnen JJ Luyben KC Kuenen JG 《Biotechnology and bioengineering》1990,36(9):921-930
Thiosphaera pantotropha is capable of aerobic heterotrophic nitrification and both aerobic and anaerobic denitrification. These phenomena have been studied in acetate-limited aerobic and anaerobic continuous cultures supplied with ammonia and nitrate. The internal reaction rates were defined, based on biochemical knowledge. The observable external conversion rates are related through a linear equation on the basis of the specified internal reaction rates. The linear equation is a Pirt relation extended for microbial systems with multiple electron donors (acetate and ammonia) and electron acceptors (oxygen and nitrate). The coefficients in this equation were estimated from the continuous culture measurements, and are composed of parameters involved in ATP production and consumption by the microorganism. It is shown that with realistic values for these parameters, the metabolically structured model describes the aerobic as well as the anaerobic experiments. 相似文献
76.
Samuel A. Kantonen Niel M. Henriksen Michael K. Gilson 《Biochimica et Biophysica Acta (BBA)/General Subjects》2018,1862(3):692-704
Background
In theory, binding enthalpies directly obtained from calorimetry (such as ITC) and the temperature dependence of the binding free energy (van't Hoff method) should agree. However, previous studies have often found them to be discrepant.Methods
Experimental binding enthalpies (both calorimetric and van't Hoff) are obtained for two host-guest pairs using ITC, and the discrepancy between the two enthalpies is examined. Modeling of artificial ITC data is also used to examine how different sources of error propagate to both types of binding enthalpies.Results
For the host-guest pairs examined here, good agreement, to within about 0.4 kcal/mol, is obtained between the two enthalpies. Additionally, using artificial data, we find that different sources of error propagate to either enthalpy uniquely, with concentration error and heat error propagating primarily to calorimetric and van't Hoff enthalpies, respectively.Conclusions
With modern calorimeters, good agreement between van't Hoff and calorimetric enthalpies should be achievable, barring issues due to non-ideality or unanticipated measurement pathologies. Indeed, disagreement between the two can serve as a flag for error-prone datasets. A review of the underlying theory supports the expectation that these two quantities should be in agreement.General significance
We address and arguably resolve long-standing questions regarding the relationship between calorimetric and van't Hoff enthalpies. In addition, we show that comparison of these two quantities can be used as an internal consistency check of a calorimetry study. 相似文献77.
Oscar C. Araújo Agatha S. Rosa Arlete Fernandes Christian Niel Cristiane A. Villela-Nogueira Vera Pannain Natalia M. Araujo 《PloS one》2016,11(4)
Hepatocellular carcinoma (HCC) is the second most common cause of cancer mortality worldwide. Most cases of HCC are associated with cirrhosis related to chronic hepatitis B virus or hepatitis C virus infections. Hypermethylation of promoter regions is the main epigenetic mechanism of gene silencing and has been involved in HCC development. The aim of this study was to determine whether aberrant methylation of RASSF1A and DOK1 gene promoters is associated with the progression of liver disease in Brazilian patients. Methylation levels were measured by pyrosequencing in 41 (20 HCC, 9 cirrhotic, and 12 non-cirrhotic) liver tissue samples. Mean rates of methylation in RASSF1A and DOK1 were 16.2% and 12.0% in non-cirrhotic, 26.1% and 19.6% in cirrhotic, and 59.1% and 56.0% in HCC tissues, respectively, showing a gradual increase according to the progression of the disease, with significantly higher levels in tumor tissues. In addition, hypermethylation of RASSF1A and DOK1 was found in the vast majority (88%) of the HCC cases. Interestingly, DOK1 methylation levels in HCC samples were significantly higher in the group of younger (<40 years) patients, and higher in moderately differentiated than in poorly differentiated tumors (p < 0.05). Our results reinforce the hypothesis that hypermethylation of RASSF1A and DOK1 contributes to hepatocarcinogenesis and is associated to clinicopathological characteristics. RASSF1A and DOK1 promoter hypermethylation may be a valuable biomarker for early diagnosis of HCC and a potential molecular target for epigenetic-based therapy. 相似文献
78.
79.
C. B. van Niel 《Archives of microbiology》1935,6(1-5):215-218
Summary It is shown that soils which fail to give Azotobacter growth when inoculated in the original elective medium may still contain viable Azotobacter cells. These develop into normal cultures when 0.00005% Na2MoO4 is added to the liquid medium.This phenomenon is explained as a result of the low molybdenum content of the soils used.In view of the possible influence of molybdenum on soil fertility the soil plaque method was tried successfully for the demonstration of a molybdenum deficiency. of soil samples. 相似文献
80.
Cirolana songkhla
sp. n. was collected from brackish-water habitats including lagoons and estuaries in the coastal zone of the lower Gulf of Thailand. C. songkhla
sp. n. is described and fully illustrated; C. songkhla
sp. n. can be recognized by the presence of abundant chromatophores dorsally, lack of ornamentation on the posterior pereonites, pleonites and pleotelson, the number of robust setae on the uropodal and pleotelson margins (uropod exopod lateral margin with 12–14 RS, mesial margin with 5–8 RS; endopod lateral margin with 8–10 RS, mesial margin with 11–13 RS; pleotelson with 12–15 RS) and lack of setae on the endopods of pleopods 3–5. A dichotomous key of brackish Cirolana species in Thailand is given. 相似文献