Analysis of repeated events

Events that may occur repeatedly for individual subjects are of interest in many medical studies. We review methods of analysis for repeated events, emphasizing that the approach taken in a given study should allow clinical questions to be addressed as directly as possible. Methods based on full models for event processes as well as on simpler 'marginal' assumptions are considered. The treatment of dependent terminating events related to the recurrent events is also discussed. We apply various methods of analysis to studies involving pulmonary exacerbations in persons with cystic fibrosis, and the occurrence of bone metastases and skeletal events in cancer patients, respectively. Most of the methodology considered can be implemented with existing software.     

Analysis of repeated measures data with clumping at zero

Longitudinal or repeated measures data with clumping at zero occur in many applications in biometrics, including health policy research, epidemiology, nutrition, and meteorology. These data exhibit correlation because they are measured on the same subject over time or because subjects may be considered repeated measures within a larger unit such as a family. They present special challenges because of the extreme non-normality of the distributions involved. A model for repeated measures data with clumping at zero, using a mixed-effects mixed-distribution model with correlated random effects, is presented. The model contains components to model the probability of a nonzero value and the mean of nonzero values, allowing for repeated measurements using random effects and allowing for correlation between the two components. Methods for describing the effect of predictor variables on the probability of nonzero values, on the mean of nonzero values, and on the overall mean amount are given. This interpretation also applies to the mixed-distribution model for cross-sectional data. The proposed methods are illustrated with analyses of effects of several covariates on medical expenditures in 1996 for subjects clustered within households using data from the Medical Expenditure Panel Survey.     

Analysis of particle fracture by repeated stressing as damage accumulation

Breakage of particulate materials by repeated low-energy stressing events is an important subject in areas such as particle transport - both mechanical and pneumatic - and comminution. Recently, a mathematical model, based on continuum damage mechanics, has been proposed to describe fracture of particles subjected to repeated loading and has been applied to model breakage of single particles by repeated impacts in a drop weight apparatus. The present paper analyzes the model in greater detail by investigating how its only parameter varies with particle size and shape, demonstrating also its validity to describe repeated impacts of particles against an anvil, such as in drop and air-gun tests. In addition, a model is proposed to describe the size distribution of the progeny under low-energy stressing events.     

“割标、改标、不三包”轮胎屡禁不止原因分析

近年来,割除商标、改换商标或不实行“三包”（包修、包换、包退）政策的轮胎在市场上大肆泛滥,有些地方甚至出现了“割标胎专卖店”。割标轮胎、改标轮胎和无“三包”轮胎的售价往往是正品胎价格的一半,因而在市场上非常走俏,有些经销商还美其名日“产品差异化营销”。各类“怪胎”的热销,增加了安全隐患,损害了消费者的利益,严重扰乱了轮胎市场正常的销售秩序。     

Analysis of a Bayesian repeated measures model for detecting differences in GP prescribing habits

A linear mixed model is used to detect a change, if any, in the prescribing habits in the UK at the general practice (family medicine) level due to an educational intervention given repeated measures data before and after the intervention and a control group. Inferences are corrected for general practice size and fundholding status. The estimates of the model parameters are obtained using Bayesian inference by applying Gibbs sampling. We develop three different priors for the parameters of the model. These three priors correspond to 'sceptical,' 'reference' and 'enthusiastic' priors in terms of the opinion about the treatment effects that they represent. We compare the results obtained by using these three priors for the parameters in the random effects model.     

Serum vitamin A during pregnancy and effects on obstetrics and perinatal outcomes in HIV infected pregnant women

To evaluate serum vitamin levels and its association with obstetrics and perinatal results in HIV infected pregnant women. Observational and prospective study carried out at Division of Infectious-Contagious Diseases in Gynecology and Obstetrics of the University Hospital, Medicine School of Ribeir?o Preto, University of S?o Paulo, involving 57 pregnant women divided into 3 groups: Group 1, with 12 normal pregnant women, it was the control group; Group 2, with 20 HIV infected pregnant women, using ZDV; and Group 3, with 25 HIV infected pregnant women, using therapy I contend ZDV, 3TC and nelfinavir. The evaluation of the serum vitamin level was obtained three times during pregnancy at equidistant time intervals and in the immediate period after birth. We also evaluated the levels of this vitamin and the hemoglobin in the blood of the umbilical cord. We obtained maternal and newborn infant anthropometric data, as well as the counting of lymphocyte TCD4 and viral load of the HIV during the pregnancy. Reduced serum vitamin levels were observed in the Group 1(25%), the Group 2(29,4%) and the Group 3(28,6%). Association was not observed between serum levels of maternal retinol and the duration of the gestation in groups 2 and 3. In groups 1 and 3, an association was observed between the maternal concentration of retinol and the newborn hemoglobin (p=0.05). In distinct way to the Control group, association was not observed between the retinol levels of the umbilical cord and the weight of the newborn in gestations of Group 2, while a trend to this association was observed in gestations of Group 3 (p=0.06). We observed high prevalence of hipovitaminosis A in the population of this study, regardless of antiretroviral scheme used.     

Polymer fractionation by repeated extraction

All current fractionation techniques—based on sol–gel equilibria—are characterized by a variation of the solvent power after each fractionation step in a discontinuous as well as in a continuous way. Overlapping of consecutive fractions is the consequence of this procedure. It can be shown that this overlapping can be diminished essentially by a multistage technique based on complete extraction at a given solvent power which then will be changed in discontinuous steps. As a further consequence, it is possible to obtain very homogeneous fractions of M?_w/M?_n < 1.03. A theoretical analysis of this technique on the basis of phase equilibrium data will be given, the results of which can be confirmed experimentally.     

Random-effects models for multivariate repeated measures

Mixed models are widely used for the analysis of one repeatedly measured outcome. If more than one outcome is present, a mixed model can be used for each one. These separate models can be tied together into a multivariate mixed model by specifying a joint distribution for their random effects. This strategy has been used for joining multivariate longitudinal profiles or other types of multivariate repeated data. However, computational problems are likely to occur when the number of outcomes increases. A pairwise modeling approach, in which all possible bivariate mixed models are fitted and where inference follows from pseudo-likelihood arguments, has been proposed to circumvent the dimensional limitations in multivariate mixed models. An analysis on 22-variate longitudinal measurements of hearing thresholds illustrates the performance of the pairwise approach in the context of multivariate linear mixed models. For generalized linear mixed models, a data set containing repeated measurements of seven aspects of psycho-cognitive functioning will be analyzed.     

反复分批式反应提高磷脂酰丝氨酸的生产能力

磷脂酶D(PLD)催化大豆磷脂合成磷脂酰丝氨酸(PS)反应是在油-水两相体系中进行的,大豆磷脂(PC)溶于有机相中,L-丝氨酸溶于水相中,但反应过程中产生副产物——胆碱,抑制酶的催化反应速率,需及时移除胆碱.本文主要采用反复分批式反应来解决这一问题.并考察了增大底物PC的浓度,PS的收率大大降低.低底物PC浓度下,添加不同浓度胆碱,转酯速率及PS收率都降低;高底物PC浓度下,采用反复分批式反应去除胆碱,转酯反应速率提高38%,PS收率达67%.研究表明反复分批式操作是一种生产磷脂的新型工艺,其中PS的生产能力明显提高,而且重复10次反应后,固定化酶活力仍保持58%,纳米SiO₂固定化磷脂酶D较好地适用于反复分批式反应.     

Deterrence of repeated oviposition in sorghum shootflyAtherigona soccata

The sorghum shootfly,Atherigona soccata, under low population density conditions lays one egg per sorghum plant. Possible regulatory mechanisms of this oviposition behavior are reported. The presence of an egg thoroughly washed and reattached to a leaf does not deter further oviposition, indicating the absence of visual cues. When washings from eggs were sprayed on sorghum plants, or when plants from which eggs had been removed were presented to a gravid female, significant deterrence was observed. Apparently a deterrent pheromone is associated with the water-soluble glue with which the females attach their eggs to the leaves. Some deterrent effect persists for at least 7 days.     

Breakage of sodium benzoate granules under repeated impact conditions

In many industries, handling or processing of relatively fragile particles takes place and predictions are required whether a significant proportion of the particles is damaged. Particles need to show good resistance against static and dynamic loads. The present paper deals with the study of particle resistance to dynamic loads (mainly the attrition behaviour) using a new instrument developed at the Delft University of Technology. The attrition test involves bombarding the particles against a flat target repeatedly, using a crank-slide mechanism. The new equipment generates extremely large number of collisions for each particle, resulting in reproducible generation of damage. The velocity is controlled by changing the frequency and amplitude of oscillation. The particles encounter many collisions on each spot on the particle surface during the test operation. The change in the particle size distribution at different intervals gives a measure of the damage caused to the particles. The damage has been characterised by estimating the specific rate of breakage, which is correlated to the frequency and amplitude of oscillation.     

产甘油假丝酵母反复分批发酵法生产甘油

研究了合成培养基和复合培养基中产甘油假丝酵母反复分批发酵法生产甘油。结果表明,当产甘油假丝酵母细胞在贫磷合成培养基、贫磷复合培养基和补充微量元素的贫磷复合培养基中分别回用13次、9次和14次时,甘油平均产量（或平均得率）的增量均超过15.0%,而甘油平均产率的增加达到37.0%以上。因此限制反复分批发酵培养基中磷含量有利于增强产甘油假丝酵母细胞合成甘油的能力。产甘油假丝酵母细胞在贫磷复合培养基中的回用次数少于贫磷合成培养基中的回用次数,其原因是贫磷合成培养基仅限制了磷源的用量,而贫磷复合培养基除限磷外,微量元素缺乏使菌体生长和甘油生产能力受到影响,回用次数减少。与传统分批发酵相比,产甘油假丝酵母反复分批发酵具有发酵周期短、不需反复培养种子、节省原料成本、形成副产物少以及节约能源动力消耗等优点,可以实现甘油高产量、高得率和高产率的相对统一,且易于放大到工业化生产水平。     

Cumulative damage of reinforced concrete subjected to repeated impact

This study was performed to develop methods of measuring the cumulative damage of steel reinforced concrete slabs subjected to repeated impact. Cumulative damage was monitored by measuring the crater depth and the reduction in ultrasonic pulse velocity across the impact region. Crater depth generally increased with increasing number of impacts and therefore was determined to be a reasonable indicator of cumulative damage. The percent reduction in velocity generally increased with increasing number of impacts up to about 40 percent or more of the total number of impacts to failure. Beyond that, interpretation of the ultrasonic results with respect to the failure mechanism appears necessary. The addition of steel fibers to the bar grid reinforcement resulted in substantial increases (about 2 to 7 times or more) in the total number of impacts to failure as compared to specimens with only bar grid or expanded metal placed at the midpoint of the slab thickness.     

Estimation of productivities in repeated fed batch cephalosporn fermentation

The theory of fed batch culture was tested on the cephalosporin C fermentation. In repeated fed batch culture a constant fraction of the culture volume is removed at fixed intervals. Since the productivity (output rate/unit volume) and specific rate of product formation may vary throughout the fed batch cycle, an “average productivity” and “average specific rate of product formation” which describe the overall behaviour of the system over several cycles are introduced. The test shows that the quasi-steady state of cephalosporin production predicted by fed batch theory can be maintained during at least 3.6 culture volume changes. The fed batch method of culture increased the productivity of the culture about 1.3 times with respect to that of simple batch culture whilst the cephalosporin C concentration was about the same in each type of culture.     

A repeated significance test in a linear model

In clinical trials we often need a sequential testing procedure for a difference between two medical treatments whose effectiveness is influenced by prognostoc factors. This article considers a repeated significance test on regression coefficients in a linear regression model. We first derive approximations for the overall significance level and power of the test and compare our test with a fixed sample test. We then discuss applications of these results to the sequential comparison of two treatments and also discuss the effect of allocation rules on the behavior of the test statistics.