The legend states Mary Worth lived a long time ago, a very beautiful young girl. One day she had a terrific accident that left her face so disfigured that nobody would look at her. She had not been allowed to see her own reflection after the accident in fear that she would loose her mind. Before this, she had spent long hours admiring her beauty in her bedroom mirror and so on...
The claim is that if you go in front of a mirror and say the words "Bloody Mary" in consecutive time intervals. Scientific Thinking Principle #4 Replicability means that a study's findings can be duplicated consistently. If one were to see a ghost, face, and or image in the mirror after consecutively saying the words "Bloody Mary" it is possible that an image may come from it. It would be highly impossible to replicate the situation in which the first "Bloody Mary" folktale situation was originated back in the 1960's or before. Scientific Thinking principle #6 Occam's Razor means if two explanations account equally for well for a phenomenon, we should generally select the more parsimonious one. It is possible that one can consecutively say the words "Bloody Mary" into a mirror and have an image appear afterwards in the mirror but it is more possible and highly likely that the house in which the person is in is haunted (if you believe in ghosts). I believe the most useful principle for evaluating this particular claim would be Occam's Razor due to the fact that so many folktales are becoming apparent this day and age.