When you use broad match, you can show for semantically similar keywords. This means that if you have the keyword laptop, you could show for the search term Alienware Gaming Computer as they are both computers or you could show for something highly related such as laptop or ultraportable.
Google does not automatically show you for all of these related terms, your budget and your Quality Score help determine what queries can trigger your ad to be displayed.
There are generally 2 pieces of data that affect how your broad match keywords get displayed for semi (non) relevant terms:
As your Quality Score increases, you’re proving to Google how relevant you are, and therefore, you can show for more queries. However, if you are highly budget capped, then you might see a few of these random queries, but not that many of them. As your budget increases, and you maintain good Quality Scores, then your keywords can be triggered by a more varied set of search terms.
For instance, if you have the keyword laptop in your account and you have a low budget and a low Quality Score, you might only show for words like laptop, new laptop, used laptop, and some misspellings. If you have a high Quality Score, then you can start to show for terms like computer, towers, Samsung computer, and more.
As your keywords show for more and more queries, then your CTR generally starts to drop as many of these queries are not that related to your ad. As you are getting these additional impressions, but at a low CTR, then your ads are often shown on the bottom of the page.
As your ads are shown on the bottom of the page, then your bottom impression share goes up and your top and absolute top impression share decline.
If you make some changes and your Quality Score goes down, then your ads show for fewer of these semi-related queries. At that point in time, your CTR goes back up, you don’t get some of these new impressions, and therefore your top impression share and absolute top impression share go up.
This is why when we are graphing our impression shares over time, we often want to look at campaigns and ad groups where we are not using broad match so that broad match isn’t polluting our trend insight data. If you are looking to find out how to create these graphs, see this article.
Broad match has a lot of good uses ranging from showing in multi-language queries to capturing a broad spectrum of search terms. However, when you are using broad match, just watch the data you are trending as broad match keywords can hide some of your deeper trends due to how it can expand and contract from showing for different search terms.