|
In time series analysis, the moving-average (MA) model is a common approach for modeling univariate time series. The notation MA(''q'') refers to the moving average model of order ''q'': : where μ is the mean of the series, the ''θ''1, ..., ''θ''''q'' are the parameters of the model and the ''ε''''t'', ''ε''''t''−1,..., ''ε''''t''−q are white noise error terms. The value of ''q'' is called the order of the MA model. This can be equivalently written in terms of the backshift operator ''B'' as : Thus, a moving-average model is conceptually a linear regression of the current value of the series against current and previous (unobserved) white noise error terms or random shocks. The random shocks at each point are assumed to be mutually independent and to come from the same distribution, typically a normal distribution, with location at zero and constant scale. ==Interpretation== The moving-average model is essentially a finite impulse response filter applied to white noise, with some additional interpretation placed on it. The role of the random shocks in the MA model differs from their role in the autoregressive (AR) model in two ways. First, they are propagated to future values of the time series directly: for example, appears directly on the right side of the equation for . In contrast, in an AR model does not appear on the right side of the equation, but it does appear on the right side of the equation, and appears on the right side of the equation, giving only an indirect effect of on . Second, in the MA model a shock affects values only for the current period and ''q'' periods into the future; in contrast, in the AR model a shock affects values infinitely far into the future, because affects , which affects , which affects , and so on forever. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Moving-average model」の詳細全文を読む スポンサード リンク
|