Dogs that tail-chase very rarely are likely to be under-represented, as videographers would have to catch the behaviour at exactly the right place and time. Conversely, dogs with clinical diagnoses may also be under-represented if owners are embarrassed (but not if they wish to raise awareness). Thus, the tail-chasing dogs on YouTube™ should approximately represent the centre of the normal distribution of dogs that chase their tails at some point in their lives. As with other survey methods, the use of video-sharing websites requires similar caution in generalizing conclusions beyond the sample population, because the populations are usually non-random and self-selecting to some extent.
http://plosone.org/article/info:doi/10.1371/journal.pone.0026553#article1.body1.sec1.p5

Just a quick note for myself