diff --git a/main.tex b/main.tex index e1b3ba4..704482f 100644 --- a/main.tex +++ b/main.tex @@ -5,18 +5,12 @@ \usepackage{ifthen} \usepackage[a4paper, landscape]{geometry} \usepackage{hyperref} -\usepackage{ccicons} \usepackage{amsmath, amsfonts, amssymb, amsthm} \usepackage{listings} \usepackage{graphicx} \usepackage{fontawesome5} \usepackage{xcolor} \usepackage{float} -\usepackage[ - type={CC}, - modifier={by-sa}, - version={3.0} -]{doclicense} \graphicspath{{./img/}} @@ -139,7 +133,6 @@ \Large{Applied Time Series } \\ \small{\href{http://vvz.ethz.ch/Vorlesungsverzeichnis/lerneinheit.view?semkez=2021S&ansicht=LEHRVERANSTALTUNGEN&lerneinheitId=149645&lang=de}{401-6624-11L}} \\ \small{Jannis Portmann \the\year} \\ - {\ccbysa} \rule{\linewidth}{0.25pt} \end{center} @@ -784,7 +777,7 @@ As with AR(p) models, there are three main steps: \end{itemize} \end{enumerate} -\subsubsection{Parameter estimation} +\subsubsection{Parameter estimation}\label{ma-parameter-estimation} The simplest idea is to exploit the relation between model parameters and autocorrelation coefficients («Yule-Walker») after the global mean $m$ has been estimated and subtracted. \\ In contrast to the Yule-Walker method for AR(p) models, this yields an inefficient estimator that generally generates poor results and hence should not be used in practice. @@ -862,8 +855,48 @@ May be more difficult in reality than in theory: \item In many cases, an AIC grid search over all $ARMA(p,q)$ with $p+q < 5$ may help to identify promising models. \end{itemize} +\subsubsection{Parameter estimation} +See \ref{ma-parameter-estimation}, with +$$E_0 = E_{-1} = E_{-2} = \dots = 0$$ +and +$$X_t = \alpha_1 X_{t-1} + \dots + \alpha_p X_{t-p} + E_t + \beta_1 E_{t-1} + \dots + \beta_q X_{t-q}$$ +respectively. + +\subsubsection{R example} +\begin{lstlisting}[language=R] +> fit0 <- arima(nao, order=c(1,0,1)); +Coefficients: + ar1 ma1 intercept + 0.3273 -0.1285 -0.0012 +s.e. 0.1495 0.1565 0.0446 +sigma^2=0.9974; log-likelihood=-1192.28, aic=2392.55 +\end{lstlisting} + +\subsubsection{Residual analysis} +See \ref{residual-analysis} again + +\subsubsection{AIC-based model choice} +In R, finding the AIC-minimizing $ARMA(p,q)$-model is convenient with the use of \verb|auto.arima()| from \verb|library(forecast)|. \\ +\vspace{.2cm} +\textbf{Beware}: Handle this function with care! It will always identify a «best fitting» $ARMA(p,q)$, but there is no guarantee that this model provides an adequate fit! \\ +\vspace{.2cm} +Using \verb|auto.arima()| should always be complemented by visual inspection of the time series for assessing stationarity, verifying the ACF/PACF plots for a second thought on suitable models. Finally, model diagnostics with the usual residual plots will decide whether the model is useful in practice. + +\section{General concepts} +\subsection{AIC} +The \textit{Akaike-information-criterion} is useful for determining the order of an $ARMA(p,q)$ model. The formula is as follows: +$$AIC = -2 \log (L) + 2(p+q+k+1)$$ +where +\begin{itemize} + \item $\log(L)$: Goodness-of-fit criterion: Log-likelihood function + \item $p+q+k+1$: Penalty for model complexity: $p, q$ are the $AR$- resp. $MA$-orders; $k = 1$ if a global mean is in use, else $0$ . The final $+1$ is for the innovation variance +\end{itemize} +For small samples $n$, often a corrected version is used: +$$AICc = AIC + \frac{2(p + q + k + 1)(p + q + k + 2)}{n - p - q - k - 2}$$ \scriptsize +\newpage + \section*{Copyright} Nearly everything is copy paste from the slides or the script. Copyright belongs to M. Dettling \\ \faGlobeEurope \kern 1em \url{https://n.ethz.ch/~jannisp/ats-zf} \\