I am using ARIMA for some time series models and some of them were not stationary and needed differencing. So then I used the differenced data for the model. I also split it for training and testing:
X = sales2['diff_1'].values size = int(len(X) * 0.70) train, test = X[0:size], X[size:len(X)]
so some of the data changed to negatives after and now when I make predictions some of the data is negative which is impossible.. do i just take the abs value or is there some procedure I am not aware of to do before making predictions?