By Shad Super in Questions —

Why Might a Search Engine Choose Not to Index a Page Even if It Has a Proper Canonical Tag Pointing to an Alternate Page?

Summary

A search engine might choose not to index a page, even if it has a proper canonical tag pointing to an alternate page, due to factors such as duplicate content, low-quality content, crawl budget limitations, lack of incoming links, and page accessibility issues. Here’s a comprehensive explanation of these reasons.

Factors Influencing Search Engine's Decision Not to Index a Page

Duplicate Content

Duplicate content refers to significant blocks of content within or across domains that match or are appreciably similar. Search engines strive to provide users with diverse and unique content. If a page's content is highly similar to another page, even with a proper canonical tag, the search engine may still choose not to index it to avoid redundancy.

Duplicate content issues and their impact are extensively discussed by Google's Search Guidelines on Duplicate Content, 2023.

Low-Quality Content

Pages with thin, low-quality content are less likely to be indexed. Search engines assess the overall quality of a page based on various criteria, including user engagement, depth of information, and relevancy. Pages that don't meet these standards even with canonical tags are not prioritized for indexing.

Google's guidelines for creating high-quality sites provide insights into what constitutes low-quality content.

Crawl Budget Limitations

A search engine’s crawl budget, which refers to the number of pages it can or wants to crawl on a site within a given timeframe, can limit the indexing of certain pages. High-priority or higher-quality pages are likely to be crawled and indexed first, leaving some pages with canonical tags unindexed if the crawl budget is exhausted.

More about crawl budgets can be found in this article by Google on Crawl Budget, 2023.

Links from other reputable websites act as endorsements for a page’s content. A page with canonical tags but few to no incoming links may be deemed less important by search engines, affecting its likelihood of being indexed.

Understanding the importance of backlinks is covered in Moz's Guide to Backlinks, 2023.

Page Accessibility Issues

Search engine bots must be able to access and crawl a page effectively to index it. Technical issues such as broken links, incorrect redirects, or blocked resources in the robots.txt file can prevent a page from being indexed, regardless of proper canonical tags.

To learn about addressing such issues, refer to the Google guide on Blocked Resources and how they impact indexing, 2022.

Specific Examples

Example 1: Internal Duplicate Content

Imagine a website with multiple pages describing similar products. If two pages about similar products have the same content, the search engine might index only one of them even if the canonical tags are correctly set.

Example 2: Thin Affiliate Pages

Pages created mainly for affiliate marketing purposes, with little unique value added, may not get indexed even if their canonical URLs point correctly. Search engines evaluate content quality and user value over canonical tags in such cases.

Conclusion

Search engines consider a variety of factors including content quality, duplicate content, crawl budget, link structure, and page accessibility when deciding whether to index a page. Proper canonical tags help but do not guarantee indexing. High-quality, unique, and accessible content will always have a better chance of getting indexed.

References