# How would I interpret this PACF and ACF graph?

I was thinking, from the ACF it looks like a nonstationary process. Or is it an AR process? From the PACF, does that mean it is AR(1)? There are 2 significant spikes in the PACF. I'm confused.

• what sort of data is it on? Oct 18 '16 at 8:39
• it is a time series data of a commodity price (natural log of real prices not nominal). Its sugar Oct 18 '16 at 9:20

## 1 Answer

Firstly, you should have integer values on the x-axis...

The ACF suggests there is long memory present in the data set. You might try to estimate it - have a look at the options.

In terms of PACF, it suggests what is the order of autocorrelation in the series, but in this case the spike is at 10th and personally, I would assume that AR(1) with $x_{t-10}$ won't really explain much. But it depends what sort of data is it on, maybe it makes sense...

• Generally noninteger values on the $x$-axis are not a problem. This could be monthly data with a period of 12, then a 1 on the axis stands for 1 year. Also, why long memory? Looks like ACF has an exponential (rather than linear) decay, hence short memory. Oct 18 '16 at 8:52
• Is there evidence of the time series being Non-stationary, judging from the acf? Oct 18 '16 at 9:07
• @user7035336 Yes, actually, there could be a unit root (and then long memory, indeed). Oct 18 '16 at 9:24
• @user7035336 if you are doing it on log prices, then I bet you made a mistake - I was gonna say you modelled the ACF on prices as they are always non-stationary ~ unit root and similar shape :) You want to take natural logarithm and then difference those to get logarithmic differences - thats' what finance works with. Try that and have a look at the ACF and PACF again. Oct 18 '16 at 11:24