# Problem with forecast se in the forecast package

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## Problem with forecast se in the forecast package

 I was experimenting with the most impressive forecast package. I tried to rig up a simplest demo of auto.arima() in action. The forecast sd looks odd to me. Here's a demo. ## Reproducibility set.seed(101) ## Simulate a series from AR(1) x <- arima.sim(list(order = c(1,0,0), ar = 0.8), n = 1000) sd(x)   # This is 1.566 which seems right ## Using ar() m1 <- ar(x) ## Use forecast::auto.arima() library(forecast) m2 <- auto.arima(x) ## How did they fare? m1 m2   # Both of them got the right model. f1 <- predict(m1, n.ahead=10) data.frame(pointest=as.numeric(f1\$pred), sd=as.numeric(f1\$se)) f2 <- forecast(m2, h=10) data.frame(pointest=cbind(as.numeric(f2\$mean), sd=(f2\$upper[,2]-f2\$lower[,2])/2)) Let me show you the two results. > data.frame(pointest=as.numeric(f1\$pred), sd=as.numeric(f1\$se))      pointest        sd 1  -0.3186451 0.9628466 2  -0.2898804 1.2264023 3  -0.2671877 1.3649788 4  -0.2492853 1.4445294 5  -0.2351619 1.4918999 6  -0.2240198 1.5206374 7  -0.2152297 1.5382519 8  -0.2082952 1.5491136 9  -0.2028244 1.5558355 10 -0.1985085 1.5600043 This seems right. The forecast sd is small at first, as we're exploiting the time series structure. But as we go deep into the future, the AR model is useless and we're down to the unconditional sd which is ~ 1.566. > f2 <- forecast(m2, h=10) > data.frame(pointest=cbind(as.numeric(f2\$mean), sd=(f2\$upper[,2]-f2\$lower[,2])/2))    pointest.V1 pointest.sd 1  -0.28090691    1.887602 2  -0.22221139    2.406792 3  -0.17578030    2.681016 4  -0.13905100    2.839174 5  -0.10999628    2.933809 6  -0.08701255    2.991505 7  -0.06883127    3.027050 8  -0.05444897    3.049081 9  -0.04307186    3.062787 10 -0.03407199    3.071333 This seems out of line. Right at n.ahead=1 the forecast sd is 1.8876 which is > 1.566. And going beyond, the forecast sd goes up to 3.07. What am I missing? -- Ajay Shah [hidden email] http://www.mayin.org/ajayshahhttp://ajayshahblog.blogspot.com        [[alternative HTML version deleted]] _______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-sig-finance-- Subscriber-posting only. If you want to post, subscribe first. -- Also note that this is not the r-help list where general R questions should go.