GoogleAlgorithmConfirmedHigh impact
Panda 4.0
Major Panda algorithm revision, not just a data refresh, which Google said affected about 7.5% of English queries.
What changed
Panda 4.0 changed the underlying quality algorithm rather than simply rerunning it over new data, and Google said roughly 7.5% of English-language queries were affected. Several large content and press-release sites recorded steep visibility losses. Google also said the revision was intended to be gentler on smaller sites that produced genuinely useful content.
Timing
Rollout began 20 May 2014. 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.