The accelerated failure time (AFT) model is most commonly used model in accelerated life testing (ALT). This model is restrictive as failure time distributions under different constant stresses differ only in terms of scale. If it is not the case, most papers on ALT use a generalization of the AFT model supposing that under different stresses not only scale but also shape parameters are different. This model has an undesirable property for accelerated experiments—the survival functions corresponding to usual and accelerated stresses intersect. In this paper, we propose flexible generalizations of the AFT model. Estimation procedures and properties of estimators are discussed. Goodness-of-fit (GOF) tests for the given models are proposed. Examples of data analysis are provided. Generalization of the results in case of step stresses is presented.

Background and objectives: Nephrocalcinosis (NC) is a common condition characterized by the deposition of calcium salts in the kidneys of very preterm infants due to tubular immaturity, intensive treatment and nutritional supplements. However, optimal vitamin D supplementation remains unclear. In most patients, NC spontaneously resolves within the first year of life, but long-term kidney function data are lacking. The aim was to study nephrocalcinosis in very preterm infants, assess risk factors and evaluate vitamin D’s impact during the first month with a 2-year follow-up. Material and Methods: This was a prospective observational study conducted over a 3-year period in infants with a gestational age of less than 32 weeks. The patients’ data were compared between the NC and control groups based on kidney ultrasound results at discharge. In the first month, the mean vitamin D intake from all sources as well as biochemical markers of calcium metabolism were collected. Patients diagnosed with NC were referred to a pediatric nephrologist after discharge. Results: NC was found in 35% of a cohort of 160 infants, more common in those with a gestational age <28 weeks. Risk factors were associated with higher morbidity and necessary treatment. At 28 days, serum 25-hydroxy vitamin D levels differed between NC and control groups (p < 0.05). The NC group with GA ≥ 28 weeks had higher vitamin D intake (p < 0.05), hypercalciuria and calcium/creatinine ratio (p < 0.01) and lower parathyroid hormone levels (p < 0.05). Follow-up showed resolution in 70% at 12 months and 90% at 24 months. Conclusions: The prevalence of NC in very preterm infants is significant, associated with lower maturity and higher morbidity. Careful vitamin D supplementation and biochemical monitoring of Ca metabolism from the first month of life should support bone health and limit the risk of nephrocalcinosis. Due to the high incidence of NC in very preterm infants, long-term follow-up is essential.

The current world crisis caused by the COVID-19 pandemic has transformed into an economic crisis, becoming a problem and a challenge not only for individual national economies but also for the world economy as a whole. The first global lockdown, which started in mid-March of 2020 and lasted for three months in Lithuania, affected the movement and behavior of the population, and had an impact on the economy. This research presents results on the impact of lockdown measures on the economy using nonparametric methods in combination with parametric ones. The impact on unemployment and salary inequality was estimated. To assess the impact of lockdown on the labor market, the analysis of the dynamics of the unemployment rate was performed using the results of the cluster analysis. The Lithuanian data were analyzed in the context of other countries, where the dynamics of the spread of the virus were similar. The salary inequality was measured by the Gini coefficient and analyzed using change point analysis, functional data analysis and linear regression. The study found that the greatest impact of the closure restrictions on socio-economic indicators was recorded in 2020, with a lower impact in 2021. The proposed multi-step approach could be applied to other countries and to various types of shocks and interventions, not only the COVID-19 crisis, in order to avoid adverse economic and social outcomes.