According to Google, share of voice (SOV) is "a relative portion of inventory available to a single advertiser within a defined market sector over a specified time period." The search giant defines "inventory" as the impression inventory available against your campaign based on your keyword and campaign settings. In addition to IS, Google has also released two related metrics: Lost IS (Rank) and Lost IS (Budget).

SOV has long been used in offline advertising. It was typically calculated as a percentage reflecting your total ad spend against your competitors' total ad spend. A more accurate measure of SOV in offline advertising would be to base the share calculation on the share of ad impressions against the target audience. Alas, this number was far more difficult to calculate, so percentage of total spending within your competitive set became the standard.

No one in the mainstream agency world ever had a satisfactory answer as to why one wouldn't adjust for efficiency. After all, if you were twice as efficient in buying media, your true SOV would be twice that of your competitive set when calculated using media spending. This is an important point to remember when using IS and the other new metrics provided by Google.

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