On Vervaat and Vervaat-error-type processes for partial sums and renewals

We study the asymptotic behaviour of stochastic processes that are generated by sums of partial sums of i.i.d. random variables and their renewals. We conclude that these processes cannot converge weakly to any nondegenerate random element of the space D[0,1]$D[0,1]$. On the other hand, we show that their properly normalized integrals as Vervaat-type stochastic processes converge weakly to a squared Wiener process. Moreover, we also deal with the asymptotic behaviour of the deviations of these processes, the so-called Vervaat-error-type processes.     

Median Relative Partial Income Polarization Indices: Investigating Economic Polarization in Poland During the Years 2005–2015

In recent years, more and more attention has been focused on the effects of economic growth and inequality changes on income polarization, as well as on the changes in the middle income class fraction. A significant part of the literature that deals with these issues is focused on polarization indices. However, the polarization indices proposed by researchers do not allow for an assessment of impact of the income distribution changes on the disappearance of the middle income class. Moreover, the general income polarization indices do not allow for assessment of polarization within the distinguished income classes. This study proposes a class of median relative polarization partial indices, which allows for a comprehensive assessment of the median relative polarization over time, within the distinguished income classes, as well as the impact of income distribution changes (its polarization or convergence) on the change of the middle income class fraction (its disappearance or increase). Using Social Diagnosis panel data (a study carried out by the Social Monitoring Council), the proposed new tool has been used to verify the hypothesis of whether changes in the household income distribution in Poland during the years 2005–2015 have led to income polarization within the three distinguished income groups—lower, middle and upper income classes. Empirical analysis shows that despite the lack of overall polarization of incomes in the household population, there was a convergence of incomes in the upper and lower income classes and polarization of incomes within the middle income class. It implies that the income distribution has not been petrified, and as on average individuals in the lower and upper income classes tend to reduce the distance to the median income, whereas the members of the middle income class tend to be pushed out of the middle class. Moreover, the flows of households into the middle income class were higher than the outflows from this class, resulting in economic convergence, i.e. changes in income distribution leading to an increase of the middle income class fraction.

     

Surrogate threshold effect: an alternative measure for meta-analytic surrogate endpoint validation

In many therapeutic areas, the identification and validation of surrogate endpoints is of prime interest to reduce the duration and/or size of clinical trials. Buyse et al. [Biostatistics 2000; 1:49-67] proposed a meta-analytic approach to the validation. In this approach, the validity of a surrogate is quantified by the coefficient of determination Rtrial2 obtained from a model, which allows for prediction of the treatment effect on the endpoint of interest ('true' endpoint) from the effect on the surrogate. One problem related to the use of Rtial2 is the difficulty in interpreting its value. To address this difficulty, in this paper we introduce a new concept, the so-called surrogate threshold effect (STE), defined as the minimum treatment effect on the surrogate necessary to predict a non-zero effect on the true endpoint. One of its interesting features, apart from providing information relevant to the practical use of a surrogate endpoint, is its natural interpretation from a clinical point of view.     

Validation of surrogate end points in multiple randomized clinical trials with failure time end points

Before a surrogate end point can replace a final (true) end point in the evaluation of an experimental treatment, it must be formally 'validated'. The validation will typically require large numbers of observations. It is therefore useful to consider situations in which data are available from several randomized experiments. For two normally distributed end points Buyse and co-workers suggested a new definition of validity in terms of the quality of both trial level and individual level associations between the surrogate and true end points. This paper extends this approach to the important case of two failure time end points, using bivariate survival modelling. The method is illustrated by using two actual sets of data from cancer clinical trials.     

Sharp bounds on the expectations of second record values from symmetric populations

Combining the greatest convex minorant approximation (Moriguti, S. (1953). A modification of Schwarz's inequality with applications to distributions. Ann. Math. Statist., 24, 107–113.) with the Hölder inequality, we establish sharp bounds on the expectations of the second record statistics from symmetric populations. We also determine the distributions for which the bounds are attained. The optimal bounds are numerically evaluated and compared with other classical rough ones.     

A Mixed Bivariate Distribution Connected with Geometric Maxima of Exponential Variables

We study the joint distribution of X and N, where N has a geometric distribution and X is the maximum of N i.i.d. exponential variables, independent of N. We present basic properties of these mixed bivariate distributions and discuss parameter estimation for this model. An example from finance, where N represents the number of consecutive positive daily log-returns of currency exchange rates, illustrates stochastic modeling potential of these laws.     

Sharp Negative Bounds On Differences of Small Quantiles and Means of Distributions from Nonparametric Families

For the distributions with decreasing density and decreasing failure rate, and decreasing density and decreasing failure rate on the average, the quantiles are less than the mean if their orders do not exceed fixed levels. We determine the sharp negative upper bounds on the differences of the small quantiles and means for the distributions from the respective families in terms of scale units generated by pth absolute central moments. We show that the bounds amount trivially to zero when p > 1, and are strictly negative for p = 1.     

On Prediction for Correlated Domains in Longitudinal Surveys

In the survey sampling estimation or prediction of both population’s and subopulation’s (domain’s) characteristics is one of the key issues. In the case of the estimation or prediction of domain’s characteristics one of the problems is looking for additional sources of information that can be used to increase the accuracy of estimators or predictors. One of these sources may be spatial and temporal autocorrelation. Due to the mean squared error (MSE) estimation, the standard assumption is that random variables are independent for population elements from different domains. If the assumption is taken into account, spatial correlation may be assumed only inside domains. In the paper, we assume some special case of the linear mixed model with two random components that obey assumptions of the first-order spatial autoregressive model SAR(1) (but inside groups of domains instead of domains) and first-order temporal autoregressive model AR(1). Based on the model, the empirical best linear unbiased predictor will be proposed together with an estimator of its MSE taking the spatial correlation between domains into account.     

On the Prediction of the Subpopulation Total Based on Spatially Correlated Longitudinal Data

In a special case of the general linear mixed model, one random component obeys a spatial autoregressive process and another a temporal autoregressive process. The population and any affiliations to subpopulations may change in time. The empirical best linear unbiased predictor is derived and may be used even if the sample size in the subpopulation is null in the period of interest. The mean squared error and its estimator are expressed. The accuracy of the predictor and the bias of the mean squared error estimator are addressed through simulations.