Definition of Impression Share
Impression Share (IS) refers to fraction of total searches for set of keywords are being shown your ads. For instance, you might have enough budget to pay for 1000 impressions per day in a PPC campaign, but there 2000 people searching every day on your target keywords. Your Impression Share would be 1000 / 2000 = 50%. In this simple case, you could double your traffic by doubling your budget. Note that you can also increase your impression share by lowering your bids, effectively spreading your budget across more, cheaper clicks.
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