First, I'd like to introduce myself, and also apologize for writing in English rather than German.
I'm Adam Lasnik, one of the guys who works on Google's Search Quality team with Matt Cutts. I trust that the owner or one of the moderators of this forum can verify that I'm a Googler from my registered google.com address

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I had the pleasure of living in Germany briefly (in Mannheim) and have gladly revisted Germany many times... but sadly, what German I had learned is now really lost to me! And undoubtedly, my German is MUCH worse than your English (unless you are talking about Hanuta, Brezeln, Doener; I remember tons of food words!)
If some of what I write is unclear, however, I'd be happy to have one of my great German-fluent colleagues translate this or future notes of mine here.
As I understand this thread, you're frustrated at how your sites' position sometimes changes starkly in our index and you'd like ways to better understand what's happening and what you can do.
So I should note some reasons why your sites' presence in our search results can change:
A) There's been a change in your sites' competitive environment
- New entrants
- Competitors have improved their Website or gotten higher quality backlinks
B) Our Googlebot has had trouble crawling your site
- Server issues on your end
- Robots.txt is blocking our crawler
- Our crawler gets stuck in a loop (often with sites that have large datasets, dynamic pages)
C) Your site has lost quality backlinks (we've devalued pages linking to you or others have removed links to your site)
D) Updates in our algorithms have changed how different aspects of your site are evaluated or weighted.
E) Your site has violated our Webmaster guidelines.
And some thoughts on what you can do:
A) Evaluate how your site can grab a competitive advantage.
Since users (and our algorithms) love unique and compelling content, it can make sense to improve the content you have on your site or add more. Blogs are one way, but you can also add reviews, extend product descriptions (or -- if your descriptions are the same as on every other site -- write your own good copy), and so on.
Also, often very simple but overlooked things can help. Making each page on your site have a unique and descriptive title tag and an appropriate (and also unique) meta description can not only help your pages' ranking, but also will provide better information for your users to bookmark and see in search results; our snippet (the description of your site in our index) is often generated from the meta description tag.
B) You can learn an enormous amount by setting up an account with our Webmaster Tools.
This is free, and doesn't even require you to make a sitemap if you prefer not to. You'll see info on how much we're crawling your site, if we're having crawl issues, what your highest PageRank pages are, and even a near-100% snapshot of all the backlinks we see for your site.
You may need to help our crawler, though, if your site's navigation is confusing or with identical content that can be found at many URLs. In these cases, it's best to restrict our Googlebot only to unique versions of pages; for instance, you may not want both your paginated articles and your printer-version to show up in our index.
C) It's possible that links that previously helped your site are no longer there or no longer passing PageRank.
By creating compelling content or useful tools, you're likely to attract more (and more quality) links naturally. But it may also be a good idea to engage in more marketing online and offline to make more people aware of your site and therefore more likely to link to you. As noted above, you can see your current backlinks in our Webmaster Tools, but we don't just show the links that "count," so some may not be passing PageRank (even when they did previously).
D) Changes in our algorithms can mean changes in your ranking.
There's not really much to be said about this, nor do about this, other than continuing to maintain a high quality site.
E) If you suspect that your site violates our guidelines -- or recently did under a different owner...
- Clean things up.
- Have a colleague or smart friend doublecheck everything.
- File a reinclusion request from within our Webmaster Tools. Though you won't receive feedback, such requests are evaluated by someone on our team usually within days to a couple of weeks, and if your request is approved, your site will likely regain its former standing.
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And a few other comments...
It's not all about PageRank.
PageRank is just one of well over 100 considerations in our current algorithms, so even though it's the only visible component, it is less uniquely critical than you might think. However, it does indeed determine whether a page is in our supplemental or main index; pages below a certain PageRank will likely be placed in the supplemental index. This is not a punishment, nor an indicator that anything is wrong with a page or a site. Pages in this index are not crawled as often as those in our main index, but will still come up for searches... and we are refreshing our supplemental index much more quickly than in the past.
"More Pulizer": Sure, one could argue that we can "set the rules," but we are opting for more transparency... sharing more information with Webmasters, giving you more tools, more ways to help us better crawl and index what's most important to you on your site. Our Webmaster Tools (along with
our new German Webmaster Blog are evidence of this... and our team that focuses on Webmaster interests is growing larger and more internationally each quarter.
"SolbachL": If we were to actually be doing or thinking as you suggest, people would search elsewhere (less AdWords revenue!) and we know it. Our entire business is built upon quality results, and we have very sizeable, international teams dedicated to this. I do hope you and others file spam reports (from within Webmaster Tools); while we are not likely to take immediate action on Webspam we find, this does help us to tune our algorithms and improve the quality of our index over time in every language.
* * *
And lastly...
I will be at Search Engine Strategies in Munich next week. In contrast with all the recent conferences I've attended, I'm not speaking at this one, so I'll have all my time to listen to presentations and also meet as many Webmasters as I can. You can find my picture across the Web and I should be easy to spot (the somewhat short, dark-haired American with "Google" on his badge) and I invite you to say hi and ask me questions.
I'm really happy that Stefanie -- an experienced German colleague of mine in Search Quality -- will be on hand to chat as well (in English or German).
I look forward to meeting many of you, and hope my (long!) note here has been helpful.