SMI gets actual ad spend from all agencies for all media. 605 gets TV viewing data with matching data rights to all that data. The calculation for ROI or ROAS is (total brand sales attributed to ads) divided by (brand ad spend).
Each new medium added to a campaign adds sales lift until saturation.
In their measurement project, the average return was $2.72. Brand A had $4.07, Brand B had $3.20, and Brand C had $1.48. Brand B bought market share by four years of increasing its share of voice. But to their calculations, it could have gotten more by using a more optimal media mix.