https://developers.google.com/search/blog/2011/05/more-guidance-on-building-high-quality
Our site quality algorithms are aimed at helping people find “high-quality” sites by reducing the rankings of low-quality content. The recent “Panda” change tackles the difficult task of algorithmically assessing website quality. Taking a step back, we wanted to explain some of the ideas and research that drive the development of our algorithms.
Below are some questions that one could use to assess the “quality” of a page or an article. These are the kinds of questions we ask ourselves as we write algorithms that attempt to assess site quality. Think of it as our take at encoding what we think our users want.
Of course, we aren’t disclosing the actual ranking signals used in our algorithms because we don’t want folks to game our search results; but if you want to step into Google’s mindset, the questions below provide some guidance on how we’ve been looking at the issue:
Writing an algorithm to assess page or site quality is a much harder task, but we hope the questions above give some insight into how we try to write algorithms that distinguish higher-quality sites from lower-quality sites.
We’ve been hearing from many of you that you want more guidance on what you can do to improve your rankings on Google, particularly if you think you’ve been impacted by the Panda update. We encourage you to keep questions like the ones above in mind as you focus on developing high-quality content rather than trying to optimize for any particular Google algorithm.
One other specific piece of guidance we’ve offered is that low-quality content on some parts of a website can impact the whole site’s rankings, and thus removing low quality pages, merging or improving the content of individual shallow pages into more useful pages, or moving low quality pages to a different domain could eventually help the rankings of your higher-quality content.
We’re continuing to work on additional algorithmic iterations to help webmasters operating high-quality sites get more traffic from search. As you continue to improve your sites, rather than focusing on one particular algorithmic tweak, we encourage you to ask yourself the same sorts of questions we ask when looking at the big picture. This way your site will be more likely to rank well for the long-term. In the meantime, if you have feedback, please tell us through our Webmaster Forum. We continue to monitor threads on the forum and pass site info on to the search quality team as we work on future iterations of our ranking algorithms.