Calculating the impact of Conversion decline

#1
On the attachment is a data table and I'm trying to understand the impact of the drop in Order Conversion from 2014 to 2013 by each Group (A-J).

I have a very basic formula for comparing Order Conversion, but I want to actually know how much the Mix (distribution) of each group is affecting the overall Order Conversion.

For example Group D represents 9.85% of total Sessions and had a drop in Conversion (-18.5%), I assume this Group would have a larger impact on Overall Conversion than Group B that only represents 0.26% of Sessions.

Is there a way to use the Mix (distribution) to weight how much the Order Conversion for each group is affecting the Overall Conversion?