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  "body": "\n## Summary\n\nThe Dickey–Fuller test is a [[statistical test|087P03NEND]] for an autoregressive time series.\nIts null hypothesis is a unit root, which means random-walk behaviour, and its alternative is stationarity.\nIts test statistic is an ordinary $t$-ratio.\nUnder the null hypothesis this ratio does not follow a Student or normal distribution, so special critical values are needed.\nDavid Dickey and Wayne Fuller introduced it in 1979, and its augmented version is a standard first step in the analysis of economic time series.\n\n## Explanations\n\n### L1 human\n\nSome series, like a stock price, wander without any pull back to a fixed level: a shock today moves them for good.\nOther series fluctuate around a level and forget shocks over time.\nThe Dickey–Fuller test asks which kind a series is, by regressing each change on the previous level.\nIf high levels tend to be followed by falls, strongly enough by the test's standard, the series is pulled back and the random-walk hypothesis is rejected.\n",
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