Rather than being a change to the overall ranking algorithm, Panda is more a new ranking factor that has been added into the algorithm

Panda is a filter that Google has designed to spot what it believes are low-quality pages. Have too many low-quality pages, and Panda effectively flags your entire site. If you have a high Panda rank that doesn’t mean that your entire site is out of Google. But what it does is adds a penalty to help ensure only the better ones make it into Google’s top results.

 

Google keeps making small algorithm changes all the time, which can cause sites to fall (and rise) in rankings independently of Panda. It also keeps updating factors that feed into the overall algorithm, such as PageRank scores, on an irregular basis. Those updates can impact rankings independently of Panda. So far, Google has confirmed when major Panda factor updates have been released. If you saw a traffic drop during one of these times, there’s a good chance you have a Panda-related problem.