Amazon shifted to a favored metric for advertisers this month, making it easier for large brands to compare ad efficiency against Google and Facebook (AMZN)

· Amazon made a metric called "return on ad spend," or ROAS, available on its ad service for sponsored ads this month.
· Until now, advertisers had to use a different metric called "advertising cost of sales," or ACOS, to track their ad effectiveness on Amazon.
· Ad agencies say the update makes it easier to compare ad...