Threshold Regression for Survival Analysis: Modeling Event Times by a Stochastic Process Reaching a Boundary

Many researchers have investigated first hitting times as models for survival data. First hitting times arise naturally in many types of stochastic processes, ranging from Wiener processes to Markov chains. In a survival context, the state of the underlying process represents the strength of an item or the health of an individual. The item fails or the individual experiences a clinical endpoint when the process reaches an adverse threshold state for the first time. The time scale can be calendar time or some other operational measure of degradation or disease progression. In many applications, the process is latent (i.e., unobservable). Threshold regression refers to first-hitting-time models with regression structures that accommodate covariate data. The parameters of the process, threshold state and time scale may depend on the covariates. This paper reviews aspects of this topic and discusses fruitful avenues for future research.Comment: Published at http://dx.doi.org/10.1214/088342306000000330 in the Statistical Science (http://www.imstat.org/sts/) by the Institute of Mathematical Statistics (http://www.imstat.org

Household retirement savings in South Africa: an analysis of pre- and post-global financial crisis determinants

This study investigates Life Cycle Hypothesis savings behaviour among South African households. The mobility matrix methodology as well as a multivariate regression analysis was employed to assess the implications of a permanent increase and a temporary decrease in household incomes based on the impacts of the global financial crisis. Using the General Household Survey data from 2002 - 2010, the study concludes that life cycle savings were greater during the period of 2002 - 2004 (,pre-financial crisis') compared with the period of 2008 - 2010 (,post-financial crisis'). Overall, the global financial crisis significantly negatively impacted household retirement savings

Thermal and spectral response characterisation of solid samples using photothermal technique

In this study, a polyvinylidene difluoride (PVDF) film was used as a pyroelectric detector in the lateral distance scanning technique to determine the thermal diffusivity of solid samples. The samples of interest were aluminuim, copper and spray paint. The values were very close to the values obtained by previous works. Next, the open photoacoustic cell (Ope) technique was used for measuring thermal diffusivity of ferrite samples. The values obtained were in the range of 0.3 cm2s- 1 to 0.5 cm2s-1. EDAX analysis proved that zinc loss happened when the sample was sintered at high temperature and the loss was accompanied by sample shrinkage and density increment. Thus, the thermal diffusivity value increased with the amount of zinc loss during the sintering process. A photopyroelectric spectroscopic (P2ES) technique was developed and applied to solid samples such as poly-methyl methacrylate doped with methylene red (PMMAMR), poly-methylmethacrylate doped with Rhodamine B (PMMA-RB) and plant leaves to see the absorption and transmission spectrum range of samples. The spectra obtained showed that they were of transmission type as predicted by Mandelis and Zver. The sample condition, that was flsLs, in obtaining the spectrum agreed with Mandelis and Zver's prediction. The trend of optical transmission spectrum fm doped PMMA-MR suggests that it could be used as a cut off filter in the range of 525nm and below to 400nm. The spectrum of PMMA-RB showed an absorption range between 500 to 550nm and a highly transmitted region above 600nm. The P2E spectra of leaf obtained were in line to the leaf absorption spectrum obtained by Lima et a1. A young leaf has a higher signal compared to a mature leaf because it is thinner and thus the optical absorption length is longer than the mature leaf. At the saturated transmission spectral region, the logarithmic signal of all these samples was found decreasing linearly with square root of chopping frequency, agreeing to Rosencwaig and Gersho's prediction. The spectrum of ferrite samples showed that the spectrum was similar to that of light source because its optical length, Ip, was still small compared to sample thickness, Ls

Expression studies of malarial antigen

Serine repeat antigen (SERA) of Plasmodium falciparum is believed to be an excellent candidate for the development of a malaria vaccine. In this study, a 22-kDa synthetic DNA fragment (SE22) from the N-terminal domain of SERA which was previously constructed by assembly polymerase chain reaction (PCR) was subcloned from the cloning vector, pCR®2.1-TOPO® into an expression vector, pPRoEX™HTa for its expression in BCG. Prior to this study, the SE22 gene was constructed in favour of mycobacterium codon usage and cloned into pCR®2.1-TOPO® to produce pNMN009. In order to express the protein, the SE22 gene was extracted from pNMN009 and subcloned into pPROEX™HTa to produce pNMN019. The expression of the fusion protein was induced with isopropylthiogalactoside (IPTG). The fusion protein was isolated and used to immunize two New Zealand white rabbits to produce polyclonal antibody against the candidate antigen. This antibody will be used to detect the expression of the synthetic SE22 gene in Mycobacterium bovis bacille Calmette Guerin

Cultivating Social Capital through Interactivity on Social Network Sites

The Internet has changed from an information tool to a social tool. More and more people use social networking sites such as Facebook to build and maintain numerous interpersonal relationships. The benefits of online social interaction can be manifested in bridging and bonding social capital. This study examined how the four dimensions of perceived interactivity (control, synchronicity, surveillance, and social bandwidth) affected users’ bridging and bonding social capital. Moreover, this study also assessed how the effects of perceived interactivity on bridging and bonding social capital were mediated by communication quality and social relationship support. This study recruited 422 respondents to participate in the survey. The first results showed that three out of four dimensions of perceived interactivity (control, synchronicity, and social bandwidth) positively influenced bridging and bonding social capital, whereas perceived surveillance negatively affected bridging social capital. Moreover, they have a stronger effect on bridging than on bonding social capital. The second findings revealed that the relationships between the two dimensions of perceived interactivity (synchronicity and social bandwidth) and bridging social capital were mediated by social relationship support

Hypergeometric series, truncated hypergeometric series, and Gaussian hypergeometric functions

In this paper, we investigate the relationships among hypergeometric series, truncated hypergeometric series, and Gaussian hypergeometric functions through some families of `hypergeometric' algebraic varieties that are higher dimensional analogues of Legendre curves.Comment: 25 page