I have data, which I am sure has a downward trend. I am trying to forecast this data using ARIMA and I want ARIMA to consider the trend when it is forecasting.
The first step in ARIMA is to determine the order of differencing for which I am using the KPSS test. The KPSS test in R has an option 'trend' and 'level'. Which one should I choose if I know that my data has a downward trend? I am confused because I dont understand what is the input ARIMA is expecting if trend has to be taken into consideration for forecasting.
Another question: Is there any sure shot way of knowing if the data follows a trend?