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I can't seem to make sense of the following results. The time-series looks more non-stationary than stationary, and when I fit an ARMA(1, 0, 0) it estimates the AR(1) term to be very close to unity (0.99). From this I would expect a simple Dickey-Fuller test with no augmented autoregressive components to either fail or just marginally succeed to reject the null of a unit root. However, the test rejects the null with a p-value of less than 0.01.

I've read all the other questions on the ADF test, but still fail to understand the intuition behind these results.

(when the ADF test is run without specifying the k-order it picks an order of 6, if that helps).

enter image description here

df <- structure(c(31.5, 29.2, 30.5, 30.5, 28.1, 27.7, 24.7, 24.3, 23, 
            19.8, 16.6, 14.9, 15.4, 13.4, 10.7, 9.4, 9.7, 8.9, 10.1, 10.3, 
            9.5, 9.4, 9.2, 8.5, 6.5, 6.3, 6.7, 6.9, 6.9, 6, 5.2, 4.5, 4.3, 
            4.7, 3.5, 3.1, 3.1, 2.8, 2.2, 1.8, 1.1, 1.8, 0.8, 1, 1.9, 0.3, 
            0.2, 0.4, 0.9, 0.9, 1, 0.9, 0.5, 1.3, 1.4, 1, 0.5, 1.3, 1.7, 
            1.7, -0.1, 0.1, 0.8, 1, 2, 2, 1.4, 2.7, 2.3, 2.4, 2, 2, 3.2, 
            2.8, 1.7, 1.4, 0.5, -0.4, 0.1, -1, -1.4, -1, -0.9, -0.9, -1.8, 
            -1.9, -1.1, -0.9, -0.8, -0.3, -0.8, -1, -0.8, -1.3, -1, -1.3, 
            -1.2, -1.2, -0.9, -0.6, 1, 1, 1.8, 2.2, 3.1, 3.1, 2.9, 2.8, 2.9, 
            3.2, 3.3, 3.2, 1.9, 2, 1.8, 2.3, 2.6, 3, 2.9, 3, 3.5, 3.4, 3.1, 
            3.4, 3.6, 3.7, 4.4, 4.2, 3.3, 3.7, 4.4, 4.6, 4, 4.4, 4.7, 4.9, 
            5, 5, 5.1, 5.5, 7.1, 7.6, 7.9, 8.2, 10, 10.9, 11.4, 11.9, 12.3, 
            12.7, 12.4, 12.2, 11.3, 10.7, 9.2, 8.5, 9.5, 8.5, 7.4, 5.9, 4.9, 
            3.9, 2.6, 2.2, 2.3, 1, 1.3, 1.2, -0.3, -0.6, -0.4, 0.2, 0.5, 
            0.9, 1.8, 1.8, 1.8, 2.6, 2.5, 3.6, 2.8, 3, 3.7, 4.4, 5, 4.8, 
            4.6, 4.4, 4.7, 4.2, 4.4, 3.5, 3.4, 3.7, 3.7, 3.3, 2.6, 2.6, 2.9, 
            3.4, 3.3, 3.2, 2.8, 2.9, 2.7, 2.3, 1.6, 1.4, 1.5, 1.3, 0.6, 0.5, 
            0.5, 0.5, 0.6, 0.5, 0.2, 0.3, 0.4, 0.3, 0.1, 0.3, 0.5, 0.3, 0, 
            0.3, 0.4, -0.1, -1.4, -1.5, -1.1, -0.6, 0, -0.2, -0.2, -1, -0.8, 
            -0.4, -0.5, -0.2, 0.7, 0.5, 0.8), .Tsp = c(1996, 2016.16666666667, 
                                                       12), class = "ts")

arima(df, order = c(1, 0, 0))

> Call: arima(x = df, order = c(1, 0, 0))
> 
> Coefficients:
>          ar1  intercept
>       0.9986    14.2496 s.e.  0.0018    13.6263
> 
> sigma^2 estimated as 0.6027:  log likelihood = -286.21,  aic = 578.42

tseries::adf.test(df, k = 0)

>   Augmented Dickey-Fuller Test
> 
> data:  df Dickey-Fuller = -5.6878, Lag order = 0, p-value = 0.01
> alternative hypothesis: stationary
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See ?adf.test:

The general regression equation which incorporates a constant and a linear trend is used...

That is, the $t$-statistic is for $H_0:\rho=1$ from the model

\begin{eqnarray*}y_t&=&\alpha+\beta t+\widetilde{y}_t\\ \widetilde{y}_t&=&\rho \widetilde{y}_{t-1}+u_t, \end{eqnarray*} Now, insert $y_t-\alpha-\beta t=\widetilde{y}_t$ in the second equation to get $$y_t-\alpha-\beta t=\rho (y_{t-1}-\alpha-\beta (t-1))+u_t$$ or \begin{eqnarray*}y_t&=&\alpha(1-\rho)+\beta t-\rho\beta (t-1)+\rho y_{t-1}+u_t\\ &=&\alpha(1-\rho)+\beta(1-\rho) t+\rho\beta +\rho y_{t-1}+u_t. \end{eqnarray*} Hence if $\rho=1$, $y_t=\beta +y_{t-1}+u_t$, a random walk with drift. If $|\rho|<1$, $y_t$ is stationary around the deterministic trend $\alpha(1-\rho)+\rho\beta+\beta(1-\rho) t$.

So, what the ADF test does here is to conclude that the series is stationary around a linear time trend, which is not entirely implausible when looking at the series.

In practice, subtract $y_{t-1}$ from either side to test the null of a zero coefficient and add lags to allow for serial correlation in $u_t$ to obtain the test regression $$ \Delta y_t=\delta+\gamma t+(\rho-1) y_{t-1}+lags+error $$

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