Impression share expresses what proportion of possible impressions your advert actually achieved. It is one of the few metrics that shows unused potential, not just the result achieved. Supplementary metrics reveal the reason for the loss: impression share lost to budget means the advert stopped running because the money ran out, while impression share lost to rank means you did not achieve sufficient quality or bid and the system did not show you. The fix differs for each cause – in the first case, increasing the budget or narrowing the targeting helps; in the second, improving the relevance of the ads and the landing page. A high impression share is not a goal in itself; for irrelevant queries, it is cheaper to sacrifice it deliberately.
See also: Ad Rank, Smart bidding (automated bidding), Budget pacing.
