I have monthly sales data for about 7 years and i need to forecast sales figures for the next 10-15 years (yearly basis, not monthly). Also i need to use ARIMA for this.
How to approach this task properly? Should i build a model for the monthly sales figures, forecast all months of needed 15 years and than aggregate it to the level of years? Or is it better to convert my monthly time series to yearly and then make a forecast (however it will be only 7 data points for the needed forecast of 15)? Or maybe some combination?