Introduction to Time Series Analysis

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Introduction to Time Series Analysis
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Stationarity in Time Series

• In time series analysis, we attempt to predict
  the future path of a variable based on
  information on its past behavior, meaning that
  the variable exhibits some regularities
• A valuable way to identify such regularities is
  through the concept of stationarity
• We say that a time series variable Yt is
  stationary if
• The variable has a constant mean at all points in
  time
• The variable has a constant variance at all
  points in time
• The correlation between Yt and Yt-k depends on
  the length of the lag (k) but not on any other
  variable

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Stationarity in Time Series

• What type of a time series variable exhibit this
  behavior?
• A variable that moves occasionally away from its
  mean (due to a random shock), but eventually
  returns to its mean (exhibits mean reversion)
• A shock in the variable in the current period
  will be reflected in the value of the variable in
  future periods, but the impact diminishes as we
  move away from the current period
• Example The variable of stock returns of Boeing
  exhibits the properties of stationarity

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Boeings monthly stock returns (1984-2003)
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Stationarity in Time Series

• A variable that does not meet one or more of the
  properties of stationarity is a nonstationary
  variable
• What is the implication of nonstationarity for
  the behavior of the time series variable?
• A shock in the variable in the current period
  never dies away and causes a permanent deviation
  in the variables time path
• Calculating the mean and variance of such a
  variable, we see that the mean is undefined and
  the variance is infinite
• Example The SP 500 index (as opposed to the
  returns on the SP index which exhibit
  stationarity)

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The SP 500 Index Exhibits Nonstationarity
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The Returns on the SP 500 Exhibit Stationarity
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The Impact of Nonstationarity on Regression
Analysis

• The major impact of nonstationarity for
  regression analysis is spurious regression
• If the dependent and explanatory variables are
  nonstationary, we will obtain high R-sq and
  t-statistics, implying that our model is doing a
  good job explaining the data
• The true reason of the good model fit is that the
  variables have a common trend
• A simple correction of nonstationarity is to take
  the first differences of variables (Yt Yt-1),
  which creates a stationary variable

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Testing for Nonstationarity

• A common way to detect nonstationarity is to
  perform a Dickey-Fuller test (unit root test)
• The test estimates the following model
• and test the following one-sided hypothesis

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Testing for Nonstationarity

• If the estimate of ?1 is significantly less than
  zero, then we reject the null hypothesis that
  there is nonstationarity (meaning that variable Y
  is stationary)
• Note The critical values of the t-statistics for
  the Dickey-Fuller test are considerably higher
  than those in the tables of the t distribution
• Example For n gt 120, the critical t-statistic
  from the tables is near 2.3, while the
  corresponding value from the Dickey-Fuller tables
  is 3.43

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Characterizing Time Series VariablesThe
Autocorrelation Function (ACF)

• The ACF is a very useful tool because it provides
  a description of the underlying process of a time
  series variable
• The ACF tells us how much correlation there is
  between neighboring points of a time series
  variable Yt
• The ACF of lag k is the correlation coefficient
  between Yt and
• Yt-k over all such pairs in the data set

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Characterizing Time Series VariablesThe
Autocorrelation Function (ACF)

• In practice, we use the sample ACF (based on our
  sample of observations from the time series
  variable) to estimate the ACF of the process that
  describes the variable
• The sample autocorrelations of a time series
  variable can be presented in a graph called the
  correlogram
• The examination of the correlogram provides very
  useful information that allows us to understand
  the structure of a time series

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Characterizing Time Series VariablesThe
Autocorrelation Function (ACF)

• Example Does the ACF of a stationary series
  exhibit a certain pattern that can be detected by
  studying the correlogram?
• For a stationary series, the autocorrelations
  between two points in time, t and tk, become
  smaller as k increases
• In other words, the ACF falls off rather quickly
  as k increases
• For a nonstationary series, this is usually not
  the case, as the ACF remains large as k increases

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Correlogram and ACF of SP Index Variable

• Note that as the number of lags (k) increases,
  the ACF declines, but at a very slow rate
• This is an indicator of a nonstationary variable
• Compare this result with the graph of the level
  of the SP Index shown previously

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Correlogram and ACF of Returns on the SP Index

• An examination of the correlogram of the variable
  of returns on the SP index shows that this
  variable exhibits stationarity
• The ACF declines very rapidly, meaning that there
  is very low correlation between observations in
  periods t and tk as k increases

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Characterizing Time Series VariablesThe
Autocorrelation Function (ACF)

• To evaluate the quality of information from the
  correlogram, we assess the magnitudes of the
  sample autocorrelations by comparing them with
  some boundaries
• We can show that the sample autocorrelations are
  normally distributed with a standard deviation of
  1/(n)1/2
• In this case, we would expect that only 5 of
  sample autocorrelations would lie outside a
  confidence interval of ? 2 standard deviations

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Characterizing Time Series VariablesThe
Autocorrelation Function (ACF)

• Given that the correlogram shows values of
  autocorrelations, these values cannot lie outside
  the interval ? 1
• As the number of time series observations
  increases above 40-50, the limits of the
  confidence interval given by the standard
  deviations become smaller
• In practical terms, if the sample
  autocorrelations lie outside the confidence
  intervals given by the correlogram, then the
  sample autocorrelations are different from zero
  at the corresponding significance level

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Correlograms and Confidence Intervals for Sample
Autocorrelations
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From Sample Data to Inference About a Time Series
Generating Model
Sample Data
Sample Autocorrelations
Population Autocorrelation
Generating Model
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Linear Time Series Models

• In time series analysis, the goal is to develop a
  model that provides a reasonably close
  approximation of the underlying process that
  generates the time series data
• This model can then be used to predict future
  values of the time series variable
• An influential framework for this analysis is the
  use the class of models known as Autoregressive
  Integrated Moving Average (ARIMA) models
  developed by Box and Jenkins (1970)

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Autoregressive (AR) Models

• In an AR model, the dependent variable is a
  function of its past values
• A simple AR model is
• This is an example of an autoregressive model of
  order 1 or an AR(1) model
• In general, an autoregressive model of order p or
  AR(p) model will include p lags of the dependent
  variable as explanatory variables

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Autoregressive (AR) Models

• Is it possible to conclude that a time series
  follows an AR(p) model by looking at the
  correlogram?
• Example Suppose that a series follows the AR(1)
  model
• The ACF of the AR(1) model begins with the value
  of 1 and then declines exponentially
• The implication of this fact is that the current
  value of the time series variable depends on all
  past values, although the magnitude of this
  dependence declines with time