T-shirt sales: aggregating

Your first attempt at forecasting wasn't great (a very high MASE of 0.97) but you know how you can improve it. You tell your manager that weekly sales are subject to a lot of variation, making it hard to predict them. Aggregating data to the quarterly level will smooth them out, leading to a more accurate forecast. You decide to do this for each of the T-shirt sizes.

If you lost progress, load the workbook 4_2_granularity.twbx from the Workbooks folder on the Desktop.