Estimation of competing risks with general missing pattern in failure types

In competing risks data, missing failure types (causes) is a very common phenomenon. In this work, we consider a general missing pattern in which, if a failure type is not observed, one observes a set of possible types containing the true type, along with the failure time. We first consider maximum likelihood estimation with missing-at-random assumption via the expectation maximization (EM) algorithm. We then propose a Nelson-Aalen type estimator for situations when certain information on the conditional probability of the true type given a set of possible failure types is available from the experimentalists. This is based on a least-squares type method using the relationships between hazards for different types and hazards for different combinations of missing types. We conduct a simulation study to investigate the performance of this method, which indicates that bias may be small, even for high proportion of missing data, for sufficiently large number of observations. The estimates are somewhat sensitive to misspecification of the conditional probabilities of the true types when the missing proportion is high. We also consider an example from an animal experiment to illustrate our methodology.     

Inference for a RPW‐Type Clinical Trial with Repeated Monitoring for the Treatment of Rheumatoid Arthritis

The present paper describes a controlled clinical trial to investigate the effect of Pulsed Electro‐Magnetic Field for the treatment of sequentially entering patients with Rheumatoid Arthritis. According to the study design, repeated monitorings of the patients are carried out to assess their clinical status. The allocation of the entering patients to either a therapy group or a placebo group is done using a newly designed adaptive allocation rule termed as randomized longitudinal play‐the‐winner rule. A discussion of some inference procedures is followed by the analysis of a real data, which shows a clear preference of the therapy over the placebo. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Two-mutation model for carcinogenesis: joint analysis of premalignant and malignant lesions.

A model for carcinogenesis that postulates two rate-limiting events for malignant transformation is a generalization of the recessive oncogenesis hypothesis, according to which inactivation of homologous tumor suppressor genes leads to cancer. This model has been shown to be consistent with a large body of epidemiologic and experimental data and has recently been used for the analysis of altered hepatic foci in rodents. These foci are considered to be premalignant lesions. In this paper the necessary mathematics for the joint analysis of premalignant and malignant lesions are developed within the framework of this model.     

Nonparametric methods for survival/sacrifice experiments

In many carcinogenicity studies, the time to disease occurrence is not clinically observable; a survival/sacrifice experiment is considered for nonparametric inference about the rate of disease occurrence. A multistate model for disease development and death is considered and an algorithm of the EM type for maximum likelihood estimation is obtained. Questions of identifiability and estimability are addressed. Under the model, interval hazards for disease occurrence are identifiable for intervals defined by the sacrifice times. A score test is developed appropriate for the comparison of two groups with respect to disease development without need of any assumption concerning lethality of the disease concerned.     

Changes in biomass and nutrient content of <Emphasis Type="Italic">Nymphoides hydrophylla</Emphasis> (Lour.) O. Kuntz. in a tropical pond: a comparison with other tropical and temperate species

A study was conducted to ascertain monthly changes in biomass of the plant and nutrient content in various organs of Nymphoides hydrophylla grown in a tropical pond during September 1999–August 2000 in relation to environmental factors. Biomass of N. hydrophylla ranged from 25 to 247 g dry weight m⁻². Among the various organs, leaf blade showed highest nitrogen (3.0–4.6%) and phosphorus content (0.9–2.4%). Comparative data of three Nymphoides species showed that N. peltata, the temperate species, had maximum potential of biomass production while long flowering period, year around growth, higher nitrogen content in various organs and presence of other associated flora were unique features of tropical species (N. hydrophylla and N. indica). Both water temperature and water level together appeared to be the best environmental variables that significantly explained the variability in biomass of N. hydrophylla.     

A generalized Luria-Delbrück model

We develop extensions of the Luria-Delbrück model that explicitly consider non-exponential growth of normal cells and a birth-death process with mean exponential or Gompertz growth of mutants. Death of mutant cells can be important in clones arising during cancer progression. The use of a birth-death process for growth of mutant cells, as opposed to a pure birth process as in previous work on the Luria-Delbrück model, leads to a large increase in the extra Poisson variation in the size of the mutant cell populations, which needs to be addressed in statistical analyses. We also discuss connections with previous work on carcinogenesis models.     

Understanding the role of starch sheath layer in graviception of Alternanthera philoxeroides: a biophysical and microscopical study

Journal of Plant Research - Plants’ ability to sense and respond to gravity is a unique and fundamental process. When a plant organ is tilted, it adjusts its growth orientation relative to...