Impression Share
Impression share doesn't tell you how visible you are in absolute terms. It tells you how much of the available visibility for you you're actually capturing, which is only useful if you already know that search market is the right one.
“Winning 90% of available impressions on a keyword none of your real customers would ever use isn't a win. It's just a figure that looks good on the wrong dashboard.”
Otto GTM ObservatoryDiagnosing the cause before reacting to the number
A low impression share typically has two distinct causes: insufficient budget to cover all available auctions, or a low Quality Score penalizing ad placement regardless of budget. Reacting to a low impression share by simply increasing the budget, without diagnosing which of the two causes is dominant, often wastes resources on a problem that actually requires improving ad or landing page relevance.
This metric should also always be cross-referenced with the commercial value of the keywords it's measured on: high impression share on low commercial-intent search terms produces no benefit, even when the figure itself looks positive.
Anti-patterns
Common mistake: increasing ad budget in response to a low impression share without checking if the real cause is an insufficient Quality Score, which no budget increase can solve alone.
Second mistake: setting a generic high-impression-share target across all account keywords, without distinguishing between high and low commercial-value terms, wasting budget to win auctions on irrelevant searches.
Practical Application
A company with a 34% impression share on its core keywords doubles its ad budget expecting doubled visibility, but impression share only rises to 41%: the real problem was a low Quality Score due to poorly relevant landing pages. After rewriting landing pages to exactly align with search queries, impression share rises to 78% with the original budget, not the doubled one.