It is difficult to build the plot, set the scenes, and all the while make a first episode in which hooks the viewers. Some great animes fail spectacularly at getting a deep enough hook in their viewers within the first episode. Through careful sampling and deliberation, it has been deduced that in order to fully get a taste of an anime's watchability or lack thereof, one must watch it until at least episode six of the series.
Sometimes things will speed up in quickly within the first couple of episodes, where intriguing mysteries can be introduced or entertaining battles can happen. However, most viewers have found that in most anime, no matter the length of the series, that episode six is the pinnacle of sampling what is to come.