Time reversibility from visibility graphs of nonstationary processes

Visibility algorithms are a family of methods to map time series into networks, with the aim of describing the structure of time series and their underlying dynamical properties in graph-theoretical terms. Here we explore some properties of both natural and horizontal visibility graphs associated to several non-stationary processes, and we pay particular attention to their capacity to assess time irreversibility. Non-stationary signals are (infinitely) irreversible by definition (independently of whether the process is Markovian or producing entropy at a positive rate), and thus the link between entropy production and time series irreversibility has only been explored in non-equilibrium stationary states. Here we show that the visibility formalism naturally induces a new working definition of time irreversibility, which allows to quantify several degrees of irreversibility for stationary and non-stationary series, yielding finite values that can be used to efficiently assess the presence of memory and off-equilibrium dynamics in non-stationary processes without needs to differentiate or detrend them. We provide rigorous results complemented by extensive numerical simulations on several classes of stochastic processes

Use of High-Nicotine/Tar-Yield (Full-Flavor) Cigarettes and Risk for Nicotine Dependence in Nationally Representative Samples of US Smokers

INTRODUCTION: The present study examines whether use of machine-estimated high-nicotine/tar-yield (full-flavor) cigarettes predicts greater risk of nicotine dependence after controlling for the influence of potential confounding factors in US nationally representative samples. METHODS: Data were obtained from multiple years of the National Survey on Drug Use and Health (NSDUH). Nicotine dependence was measured by (1) the Nicotine Dependence Syndrome Scale and (2) latency to first cigarette after waking. Associations between use of high-nicotine/tar-yield cigarettes and risk for nicotine dependence were examined using multiple logistic regression. RESULTS: The odds of nicotine dependence were reliably greater among users of high- compared to lower-nicotine/tar-yield cigarettes even after adjusting for sociodemographic and other smoking characteristics (Ps < .0001). This relationship was (1) generally graded across differing nicotine/tar-yield cigarettes, (2) discernible across two definitions of nicotine dependence and multiple NSDUH survey years, and (3) observed among adult and adolescent smokers. CONCLUSION: Use of high-nicotine/tar-yield cigarettes is associated with increased odds of nicotine dependence, a relationship that has important tobacco regulatory implications. Whether the widespread marketing and availability of high-nicotine/tar-yield cigarettes is increasing risk of nicotine dependence among US smokers warrants further research. IMPLICATIONS: This study adds additional empirical evidence to the relation of machine measured high-yield cigarettes and likelihood of nicotine dependence, and draws some implications in regards to regulation