Panda content quality system
Site-wide content quality classifier that demoted thin, duplicated and low-value pages; launched February 2011 and later folded into core ranking.
What changed
Panda applied a machine-learned quality score at site level, so a large share of low-value pages could suppress rankings across an entire domain. The first release affected roughly 12% of US English queries and hit content farms, scraper sites and thin aggregators hardest. Google published a list of questions site owners should ask about content quality as its official guidance. Panda ran as a separate periodically refreshed filter until it was incorporated into the core ranking algorithm in January 2016.
Timing
Rollout began 24 February 2011. No completion date was announced. Rankings typically remain unstable for the whole rollout window, so measurements taken inside it are not a reliable read on the outcome.