The ubiquity of mobile devices led to a surge in intensive longitudinal (or time series) data of individuals. This is an exciting development because personalized models both naturally tackle the issue of heterogeneities between people and increase the validity of models for applications. A popular model for time series is the Vector Autoregressive (VAR) model, in which each variable is modeled as a linear function of all variables at previous time points. A key assumption of this model is that the parameters of the true data generating model are constant (or stationary) across time. The most straightforward way to check for time-varying parameters is to fit a model that allows for time-varying parameters. In the present paper we compare two methods to estimate time-varying VAR models: the first method uses a spline-approach to allow for time-varying parameters, the second uses kernel-smoothing. We report the performance of both methods and their stationary counterparts in an extensive simulation study that reflects the situations typically encountered in practice. We compare the performance of stationary and time-varying models and discuss the theoretical characteristics of all methods in the light of the simulation results. In addition, we provide a step-by-step tutorial for both methods showing how to estimate a time-varying VAR model on an openly available individual time series dataset.
Several implementations of related models are available as free software packages. The R-package implements the estimation of a time-varying AR model using a rolling window approach. The R-package implements the estimation of (time-varying) state-space models, of which the time-varying VAR model is a special case. While the state-space model framework is very powerful due to its generality, it requires the user to specify the way parameters are allowed to vary over time, for which often no prior theory exists in practice . In parallel efforts @cite_9 developed the R-package , which estimates time-varying AR and VAR models using kernel smoothing similarly to kernel-smoothing approach described in the present paper. An interesting way to modeling time-varying parameters is by using the fused lasso . However, to our best knowledge there are currently only implementations availabe to estimate time-varying Gaussian Graphical Models with this type of method: a Python implementation of the SINGLE algorithm and a Python implementation of the (group) fused-lasso based method as presented in @cite_3 .
@cite_9 @cite_3
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Compressed sensing (CS) lowers the number of measurements required for reconstruction and estimation of signals that are sparse when expanded over a proper basis. Traditional CS approaches deal with time-invariant sparse signals, meaning that, during the measurement process, the signal of interest does not exhibit variations. However, many signals encountered in practice are varying with time as the observation window increases (e.g., video imaging, where the signal is sparse and varies between different frames). The present paper develops CS algorithms for time-varying signals, based on the least-absolute shrinkage and selection operator (Lasso) that has been popular for sparse regression problems. The Lasso here is tailored for smoothing time-varying signals, which are modeled as vector valued discrete time series. Two algorithms are proposed: the Group-Fused Lasso, when the unknown signal support is time-invariant but signal samples are allowed to vary with time; and the Dynamic Lasso, for the general class of signals with time-varying amplitudes and support. Performance of these algorithms is compared with a sparsity-unaware Kalman smoother, a support-aware Kalman smoother, and the standard Lasso which does not account for time variations. The numerical results amply demonstrate the practical merits of the novel CS algorithms. The vector autoregressive (VAR) model is a powerful tool in learning complex time series and has been exploited in many fields. The VAR model poses some unique challenges to researchers: On one hand, the dimensionality, introduced by incorporating multiple numbers of time series and adding the order of the vector autoregression, is usually much higher than the time series length; On the other hand, the temporal dependence structure naturally present in the VAR model gives rise to extra difficulties in data analysis. The regular way in cracking the VAR model is via "least squares" and usually involves adding different penalty terms (e.g., ridge or lasso penalty) in handling high dimensionality. In this manuscript, we propose an alternative way in estimating the VAR model. The main idea is, via exploiting the temporal dependence structure, formulating the estimating problem to a linear program. There is instant advantage of the proposed approach over the lasso-type estimators: The estimation equation can be decomposed to multiple sub-equations and accordingly can be solved efficiently using parallel computing. Besides that, we also bring new theoretical insights into the VAR model analysis. So far the theoretical results developed in high dimensions (e.g., Song and Bickel, 2011 and Kock and Callot, 2015) are based on stringent assumptions that are not transparent. Our results, on the other hand, show that the spectral norms of the transition matrices play an important role in estimation accuracy and build estimation and prediction consistency accordingly. Moreover, we provide some experiments on both synthetic and real-world equity data. We show that there are empirical advantages of our