I have used NIST data to calculate ACF in excel which worked fine and coded in our programming language (NOT R).
Here is the plot of ACF:
Now my questions are:
1) From this ACF series how can I we find out this is stationary or not (By looking at values programatically without plotting)? Is there any statistical approach like if X number of values fall out of some MAX_VALUE, then this series fails ACF stationary check or something like that?
2) I have observed that sometimes people using "differenced" series to calculate ACF instead of using original series. Can you help us with when to use "differenced" series and when to use "original" series to calculate ACF?
Thanks for your time!