This is the percentage of time your ad didn't appear because your budget is too low. Depending on the budget delivery method you've set for a campaign, your ads may appear in the morning, but then your daily budget runs out and prevents you from bidding in the afternoon. Or you may miss opportunities for impressions evenly throughout the day. In either case, a value of 20% indicates that in 20% of customer searches that matched your keywords, your ad didn't appear because your budget is too low. 

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