tailieunhanh - Econometric theory and methods, Russell Davidson - Chapter 14

Chapter 14 Unit Roots and Cointegration Introduction In this chapter, we turn our attention to models for a particular type of nonstationary time series. For present purposes, the usual definition of covariance stationarity is too strict. We consider instead an asymptotic version | Chapter 14 Unit Roots and Cointegration Introduction In this chapter we turn our attention to models for a particular type of non-stationary time series. For present purposes the usual definition of covariance stationarity is too strict. We consider instead an asymptotic version which requires only that as t 1 the first and second moments tend to fixed stationary values and the covariances of the elements yt and ys tend to stationary values that depend only on t s . Such a series is said to be integrated to order zero or I 0 for a reason that will be clear in a moment. A nonstationary time series is said to be integrated to order one or I l 1 if the series of its first differences Ayt yt y 1 is I 0 . More generally a series is integrated to order d or I d if it must be differenced d times before an I 0 series results. A series is I 1 if it contains what is called a unit root a concept that we will elucidate in the next section. As we will see there using standard regression methods with variables that are I 1 can yield highly misleading results. It is therefore important to be able to test the hypothesis that a time series has a unit root. In Sections and we discuss a number of ways of doing so. Section introduces the concept of cointegration a phenomenon whereby two or more series with unit roots may be related and discusses estimation in this context. Section then discusses three ways of testing for the presence of cointegration. Random Walks and Unit Roots The asymptotic results we have developed so far depend on various regularity conditions that are violated if nonstationary time series are included in the set of variables in a model. In such cases specialized econometric methods must be employed that are strikingly different from those we have studied 1 In the literature such series are usually described as being integrated of order one but this usage strikes us as being needlessly ungrammatical. Copyright 1999 Russell Davidson .

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