Dickey–Fuller test

Dickey–Fuller test is a statistical test aimed at testing for a unit root in a time series sample. It is named after the statisticians David Dickey and Wayne Fuller, who developed the test in the late 1970s. The presence of a unit root indicates that the time series is non-stationary and possesses a structure that could potentially affect the reliability of some statistical models when applied to the data. The Dickey–Fuller test is crucial in econometrics and time series analysis for distinguishing between stationary and non-stationary time series.

Overview

The Dickey–Fuller test starts with the following null hypothesis: the time series has a unit root, meaning it is non-stationary. It contrasts this with the alternative hypothesis that the time series does not have a unit root and is therefore stationary. The test uses a specific form of regression to estimate the parameters and then computes a test statistic, which is compared to critical values for the Dickey–Fuller test. If the test statistic is less than the critical value, the null hypothesis is rejected, suggesting the time series is stationary.

Types of Dickey–Fuller Tests

There are several variations of the Dickey–Fuller test, each designed for different types of time series data:

• Augmented Dickey–Fuller (ADF) test: This version extends the basic Dickey–Fuller test to include lagged differences of the series. This addition helps to account for higher-order correlation structures within the time series, making the ADF test more widely applicable.

• Phillips–Perron (PP) test: While not a Dickey–Fuller test per se, the PP test is often mentioned alongside it because it also tests for unit roots but corrects for serial correlation and heteroskedasticity in the error terms without adding lagged difference terms.

Test Equation

The basic Dickey–Fuller test equation is:

\[ \Delta y_t = \alpha + \beta t + \gamma y_{t-1} + \delta \Delta y_{t-1} + \epsilon_t \]

where:

• \( \Delta \) is the difference operator,
• \( y_t \) is the value of the time series at time \( t \),
• \( \alpha \) is a constant,
• \( \beta \) represents a time trend,
• \( \gamma \) is the coefficient on \( y_{t-1} \), the lagged value of the series, and
• \( \epsilon_t \) is the error term.

Application

The Dickey–Fuller test is widely used in the fields of economics and finance to test for the presence of a unit root in time series data such as GDP, stock prices, and interest rates. Its application is crucial for the correct specification of econometric models, as many standard statistical techniques assume stationarity of the underlying data.

Limitations

While the Dickey–Fuller test is a powerful tool for detecting non-stationarity, it has limitations. It may have low power against certain alternatives, meaning it might not always distinguish between a unit root and a stationary process close to being non-stationary. Additionally, the choice of lag length in the ADF version of the test can significantly affect the test's outcome.

See Also

• Time series analysis
• Stationary process
• Unit root
• Augmented Dickey–Fuller test
• Phillips–Perron test