The dickey fuller test results can change radically depending on how many lags you use. If you don't use enough lags, then the regression assumptions won't hold and your test is not correct. If you use too many lags, then you lose a lot of power. There is a lot of literature on how many lags to use. Schwert developed an algorthm and it's in eric zivot's splus finmetrics text but I don't have that book with me at the moment. There's also the question of whether to include a trend or intercept or both in your model. This issue is discusssed very clearly in hamilton.
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Megh Dal Sent: Thursday, August 16, 2007 6:58 AM To: r-help@stat.math.ethz.ch Subject: [R] ADF test Hi all, Hope you people do not feel irritated for repeatedly sending mail on Time series. Here I got another problem on the same, and hope I would get some answer from you. I have following dataset: data[,1] [1] 4.96 4.95 4.96 4.96 4.97 4.97 4.97 4.97 4.97 4.98 4.98 4.98 4.98 4.98 4.99 4.99 5.00 5.01 [19] 5.01 5.00 5.01 5.01 5.01 5.01 5.02 5.01 5.02 5.02 5.03 5.03 5.03 5.03 5.03 5.04 5.04 5.04 [37] 5.04 5.04 5.04 5.05 5.05 5.06 5.06 5.06 5.07 5.07 5.07 5.07 5.08 5.07 5.08 5.08 5.09 5.10 [55] 5.10 5.09 5.10 5.10 5.10 5.10 5.10 5.10 5.10 5.10 5.11 5.11 5.11 5.11 5.11 5.11 5.11 5.12 [73] 5.12 5.12 5.12 5.13 5.14 5.14 5.14 5.14 5.14 5.15 5.15 5.15 5.15 5.14 5.15 5.15 5.15 5.16 [91] 5.16 5.16 5.16 5.16 5.16 5.16 5.16 5.16 5.16 5.16 5.17 5.17 5.17 5.17 5.17 5.18 5.18 5.18 [109] 5.18 5.18 5.19 5.19 5.20 5.20 5.20 5.20 5.20 5.21 5.21 5.21 5.21 5.21 5.21 5.22 5.22 5.23 [127] 5.23 5.23 5.23 5.24 5.24 5.24 5.25 5.24 5.24 5.25 5.26 5.26 5.26 5.26 5.26 5.26 5.26 5.27 [145] 5.27 5.26 5.27 5.27 5.28 5.29 5.29 5.29 5.29 5.30 5.30 5.30 5.31 5.31 5.31 5.32 5.32 5.33 [163] 5.33 Now I want to conduct a test for stationarity using ADF test : > adf.test((data[,1]), "stationary", 0) Augmented Dickey-Fuller Test data: (data[, 1]) Dickey-Fuller = -3.7351, Lag order = 0, p-value = 0.02394 alternative hypothesis: stationary But surprisingly it leads towards rejestion of NULL [p-value is less than 0.05], i.e. indicates a possible stationary series. However ploting a graph of actual data set it doesn't seem so. Am I making any mistakes ? Can anyone give me any suggestion? Regards, Megh --------------------------------- [[alternative HTML version deleted]] ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. -------------------------------------------------------- This is not an offer (or solicitation of an offer) to buy/se...{{dropped}} ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
