I am working on a project where I predict the total quantities sold at the ITEM/DAY leve. As for the model, I decided to with an ARIMA model (I'm using R). For guidance, I decided to follow the two tutorials below:
The first one here
The second one here
The thing that I am confused about is that, in the second one, they split the data into training and testing and they fit the model on the training set and did the evaluation on the test set (all that makes sense). However, in the first article, they didn't do the splitting.
Can anyone guide me through which is the correct approach and provide me with an explanation?