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I have a question on a ACF/PACF result reported below.

Using a daily time series composed by 953 values I have the following result plotting the acf and pacf results.

enter image description here

My questions are:

  1. how can I interpret the fact that in the ACF there are a kind of pick at lag 7, 14, 21 ,ecc ? It seems drop down only at 600th lag

  2. how can I interpret the PACF result due to this stange behavior ?

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This would indicate a clear seasonal effect of period 7 (such as you might see with a "day-of-week" type effect and daily data)

The fact that it extends for a long time suggests you might need seasonal differencing.

If you look at the plots for seasonally differenced series you'll probably see indication of a small ARMA model -- perhaps an MA(1), say.

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