About ad rotation settings
Ad rotation is the way we deliver your ads on both the Search Network and the Display Network. If you have multiple ads within an ad group, your ads will rotate because no more than one ad from your account can show at a time. Use the ad rotation setting to specify how often you'd like the ads in your ad group to be served relative to one another.
This article explains the 4 settings for ad rotation.
Ad rotation settings
Optimize for clicks (default)
The optimize for clicks setting gives preference to ads that are expected to attract more clicks than other ads in the ad group, based on your past clickthrough rates (CTRs). With this setting, your ad group will likely receive more impressions and clicks overall, since higher-quality ads attain better positions and attract more user attention. For video campaigns, this setting is called optimize for views and works the same way.
How it works: As data is accumulated, ad serving will become weighted more heavily in favor of the ads that appear statistically likely to perform better. Ads expected to attract more clicks are delivered more often into the ad auction than other ads in the ad group. These ads show more often, resulting in higher ad-served percentages. However, it's possible for ad serving to remain relatively even within this setting, if ads in an ad group have similar performance or if the ad group does not receive a substantial amount of impressions and clicks for some period of time.
Keep in mind: The ad with the highest CTR may not always be the one expected to get the most clicks. That’s because the number of clicks an ad is expected to get is also affected by how often an ad is eligible to enter an auction. Expected CTR is only one factor of Ad Rank, which also considers landing page experience and ad relevance (among other factors) when determining your ad’s position and whether it will show at all. So a more relevant ad with a better landing page experience but a lower expected CTR could be eligible to show on search results more often. That can, in turn, lead to a higher overall number of expected clicks.
Optimize for conversions
The optimize for conversions setting gives preference to ads that are expected to provide more conversions, like purchases and sign-ups. Those ads are delivered more often into the ad auction. Although the optimize for conversions setting may result in your ad group receiving fewer clicks than the previous option, it's more likely to receive more conversions, which can result in an improved return on investment.
How it works: This option takes both clickthrough rate (CTR) and conversion rate into consideration. If there isn't enough data to determine which ad will provide the most conversions, ads will rotate using optimize for clicks data.
Keep in mind: Your ad group might receive fewer clicks, but it's likely to receive more conversions, which can result in an improved return on investment. If there isn't enough conversion data to determine which ad is likely to provide the most conversions, ads will rotate to optimize for clicks.
The rotate evenly setting delivers your ads more evenly into the ad auction. In general, rotating evenly allows ads with lower clickthrough rates and conversion rates to show more often, so this option could result in a lower average position or fewer clicks and conversions.
How it works: When a new ad group is enabled, or an ad in that ad group is added or changed, we’ll rotate the ads in the ad group evenly for a 90-day period. After the 90-day period, the campaign will automatically begin to optimize for clicks or conversions. If the campaign is using a conversion-based bidding strategy such as Target CPA bidding, Enhanced CPC, Target CPA, or Target ROAS, it will optimize for conversions, otherwise it will optimize for clicks.
Even though you select an ad rotation setting at the campaign level, the even rotation period is tracked separately for each ad group. It starts (and resets) for an ad group whenever the ads in that ad group change: specifically, when a new ad gets added, when an existing ad is changed, or when a paused/deleted ad is enabled.
When the even rotation period ends and we optimize for clicks, the campaign setting will continue to say "Rotate evenly." Note that for campaigns using the vCPM bidding option, ads will continue to rotate more evenly beyond the 90 days.
Like the rotate evenly setting, the rotate indefinitely setting delivers your ads more evenly into the ad auction, but does so for an indefinite amount of time and does not optimize.
Because this option allows lower-performing ads to run as often as higher-performing ads for an indefinite period of time, this option is not recommended for most advertisers.
Keep in mind
With the rotate evenly and rotate indefinitely options, the percentage of impressions for ads served in the ad group will be more even than the other ad rotation options. However, because the quality of the ads differ, and quality is used to determine where an ad shows, or if it even shows at all, the percentage of impressions for each ad may not be perfectly even. An ad with high quality may appear on the first page of search results, while an ad with low quality may show on the second page of search results, which reaches a smaller number of users.