Where does it make sense to run extensions? For example, Sitelinks, Call, etc.
Sitelinks are fantastic--they take up precious pixels that your competitors would love to have, and don't cost anything extra to run. However, there's a dirty secret to them--they're actually terrible for your best performing campaigns. This is because the highest converting campaigns are probably based around low-funnel (usually product-specific) ads, and you've carefully selected the best landing page for someone searching for that product. If they click on one of these Sitelinks, then they've hopped up a notch or two in the funnel, and are a step or more further from converting.
However, in higher-funnel campaigns, like category-level keywords, then Sitelinks can be a great way to do the opposite--provide the shopper with a few specific examples that fall under the broad search they'd made.
Call extensions are also tricky.
If you're using Google's call tracking, for example, you can't actually tell which orders are placed from customers who used those forwarding numbers--you can only tell that the call was placed. Note that Google is set up to encourage you to treat calls as CONVERSIONS, not as CLICKS. This means that you need to keep very close tabs on your phone-call conversion rate, so that you can extrapolate what the revenue impact of those calls might be. However, if the call quality (revenue-speaking) is significantly different from the rest of your call volume, then this will be misleading.
Call extensions can, however, play well into a specific mobile-targeted campaign strategy, say, for brick and mortar stores, where direct attribution is already abstracted.
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