Why, when having died or having been killed, do some Jedis (and or Siths) fade away while some don't?

For instance, Qui Gon Jin had to be burned after he died as well as Darth Vader, but Yoda and Obi Wan both disappeared when they died. I don't think you have to be some sort of master because Vader was trained by Darth Sideous(the Emperor) for years so he must have reached a "master" level.Who knows?