I have a time series data for shipment with following variables:
- Year: 2008, 2009, 2010, 2011, 2012, 2013
- Month: jan, feb, ..., dec
- Number of ordering days
- Shipment Volume
I want to know the effect of the number of ordering days on the shipment volume.
Can this can be done using regression method? If so, should I consider month and year as categorical variables?