Google has confirmed to Search Engine Land that earlier this month it did update its core algorithm in regards to how it measures content quality.
Speculation from webmasters and publishers affected by this update had ranged from a Panda or Penguin update, to a ‘Phantom’ update targeting “how-to” posts.
Although Google didn’t provide specific details on the changes, they have in the past published general guidelines for high-quality sites which included:
If you’ve noticed a previously unexplained drop-off in traffic this month, it could be due to the changes in how Google assesses quality content. Review your content using a filter of Google’s ‘quality’ guidelines and consider upgrading or removing substandard content.