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When Google Blog Search crawls a page, can it tell whether a link is in the article or whether it's in a comment?

If it can, does it grade the links, that is, a link from a blog comment is only going to be worth 50% of what an actual link inside the article would be?

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In principle, there's no difference between the value of a link regardless of where it appears in a page (I mean, apart from logic specific to each crawler). Update: as Mike Hudson showed in his answer, different sections of a page are valued differently, but I can't say for sure how well-recognized the "blog" pattern is - between many more forms a website can take. A crawler specialized in blogs, however, is likely to perform well in this regard.

Anyway, in the specific case of blog comments (or other tools that accept lots of anonymous user content), most softwares automatically add the rel="nofollow" attribute to each link that appears in a comment (mostly to prevent abuse). As a result, those links do not influence the rank of the target site when search engines crawl it.

Quoting the Wikipedia article linked above:

nofollow is a value that can be assigned to the rel attribute of an HTML a element to instruct some search engines that a hyperlink should not influence the link target's ranking in the search engine's index. It is intended to reduce the effectiveness of certain types of search engine spam, thereby improving the quality of search engine results and preventing spamdexing from occurring.

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I'd have to disagree with your statement "there's no difference between the value of a link regardless of where it appears in a page" - see my answer below –  Mike Hudson Jun 4 '12 at 1:59
    
@MikeHudson you're right, while I knew (from my text mining background) that different areas of a document have different significance, I was unsure whether or not crawlers took that into account (or how good they were in doing that), so I generically said "logic specific to each crawler". But your links confirm they in fact do that well. –  mgibsonbr Jun 4 '12 at 2:18
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Yes, search engines can tell different areas of content from another. In addition they can tell what links come from what section and can apply weighting to those areas.

Information Retrieval science is a complex area, but the there have been some interesting studies by the search engine scientists to help with identifying page-level segmentation analysis:

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This is all interesting–really–from a theoretical perspective, but whitepapers and patents don't mean implementation. (Remember that the general context for questions here is practical considerations.) For purposes of the question, and reinforcing your examples, do you have any references pointing out this stuff is actually being used, eg. one of Cutts' Q&A videos, etc.? –  Su' Jun 4 '12 at 9:27
    
@Su' the best I can do is this video from Cutts - youtube.com/watch?v=LHZIt0FisWI –  Mike Hudson Jun 5 '12 at 12:39
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Recognizing different sections on pages is definitely possible in some cases. However, trying to determine the weight of links in those sections is over-thinking it in my opinion. If you're a blog owner, and people post links in their comments, then you can add the rel=nofollow if you don't want to vouch for those links. Past that, keep in mind that comments can also be very important content (including their links). –  John Mueller Jun 7 '12 at 6:53
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