Nonparametric trending regression with cross-sectional dependence
Introduction
Much econometric modelling of nonstationary time series employs deterministic trending functions that are polynomial, indeed frequently linear. However the penalties of mis-specifying parametric functions are well appreciated, and nonparametric modelling is increasingly widely accepted, at least in samples of reasonable size. The inability of polynomials to satisfactorily globally approximate general functions of time deters study of polynomial functions whose order increases slowly with sample size, and rather leads one to consider the possibility of a smooth trend mapped into the unit interval and approximated by a smoothed kernel regression. For example, Starica and Granger (2005) employed this approach in modelling series of stock prices. There is a huge literature on such fixed-design nonparametric regression, principally in the setting of a single time series.
Here we are concerned with panel data, where series of length have a common, nonparametric, time trend but also additive, fixed, individual effects, for which we have to correct before being able to form a trend estimate. We assume an asymptotic framework in which is large, but not necessarily , so that the cross-sectional mean at a given time point is not necessarily consistent for the trend, hence the recourse to smoothed nonparametric regression. A major feature of the paper is concern for possible cross-sectional correlation and/or heteroscedasticity. These influence the asymptotic variance of our trend estimate, and thence also the mean squared error and consequent optimal rules for bandwidth choice. The availability of cross-sectional data enables us to propose a trend estimate, based on the generalized least squares principle, that reduces the asymptotic variance. This estimate, along with its asymptotic variance (and that of the original trend estimate), depends on the cross-sectional covariance matrix. In general this is not wholly known, and possibly not known at all. Using residuals from the fitted trend, we consistently estimate its elements, so as to obtain a feasible improved trend estimate, and a consistent estimate of its variance, as well as feasible optimal bandwidths that are asymptotically equivalent to the infeasible versions. These results are valid with remaining fixed as increases, and they continue to hold if is also allowed to increase, in which case there is a faster rate of convergence, and in this latter situation our results hold irrespective of whether or not the covariance matrix is finitely parameterized. Peter Phillips has made seminal contributions to research in both panel data and nonparametric estimation, among other areas.
Section 2 describes the basic model. In Section 3 we present a simple trend estimate and its mean squared error properties. Improved estimation is discussed in Section 4. In Section 5 optimal bandwidths are reported. Section 6 suggests estimates of the cross-sectional covariance matrix, with asymptotic justification for their insertion in the optimal bandwidths and improved trend estimates. Section 7 suggests some directions for further research. Proof details may be found in two appendices.
Section snippets
Panel data nonparametric model
We observe , generated by where the and are unknown constants, and the are unobservable zero-mean random variables, uncorrelated and homoscedastic across time, but possibly correlated and heteroscedastic over the cross section. Thus we impose
Assumption 1 For all , for all there exist finite constants such that and for all ,
Our focus is on estimating the time trend. Superficially this is represented in
Simple trend estimation
We introduce a kernel function , satisfying and a positive scalar bandwidth . Then with the abbreviation define the estimate
Important measures of goodness of nonparametric estimates, which lead to optimal choices of bandwidth , are mean squared error, i.e. and mean integrated squared error, i.e.
To approximate these we require conditions on and .
Assumption 2 is
Improved trend estimation
Improved estimation of the trend requires it to be identified in a different way from that in Section 2, in particular to shift its location. Consider the representation where the bracketed superscript represents a vector of weights, such that where is a vector of 1’s. This represents a generalization of (2.1), (2.5), in which . It is convenient to write (4.1), for , in -dimensional column vector form as
Optimal bandwidth choice
A key question in implementing either or is the choice of bandwidth . Choices that are optimal in the sense of minimizing asymptotic MSE or MISE are conventional. The following theorem differs only from well known results in indicating the dependence of the optimal choices on and , and so again no proof is given. Theorem 3 Let Assumption 1, Assumption 2, Assumption 3, Assumption 4, Assumption 5 hold. The minimizing asymptotic MSE and MISE of are respectively
Feasible optimal bandwidth choice and trend estimation
In practice the optimal bandwidths of the previous section cannot be computed. The constants and are trivially calculated, but and are unknown. Discussion of their estimation can be found in the nonparametric smoothing literature, see e.g. Gasser et al. (1991), and there is nothing about our setting to require additional treatment here, apart from the improved estimation possible by averaging over the cross section. More notable is the need to approximate the partly or wholly unknown
Monte Carlo study of finite sample performance
As always when large sample asymptotic results are presented, the issue of finite-sample relevance arises. In the present case, one interesting question is the extent to which matches the efficiency of , and whether it is actually better than , given the sampling error in estimating . We study this question by Monte Carlo simulations in the case where has the factor structure (4.19).
In (2.1), we thus take where the and have mean zero and variance 1, and
Further directions for research
- 1.
Our asymptotic variance formulae for and appear also in central limit theorems, under some additional conditions, indeed one could develop joint central limit theorems for both and at finitely many, , fixed frequencies , with asymptotic independence across the . When the bias is negligible relative to the standard deviation, the convergence rate will be when is fixed, and faster if is allowed to increase with . We could also develop a central limit
Acknowledgements
This research was supported by ESRC Grant RES-062-23-0036. I am grateful for the helpful comments of a referee and Jungyoon Lee, and to the latter also for carrying out the simulations.
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