Eyewitness testimony is generally presumed to be more reliable than circumstantial evidence. Studies have shown, however, that individual, separate witness testimony is often flawed, and parts of it can be meaningless. That can occur because of flaws in eyewitness identification (such as faulty observation and recollection, or bias) or because a witness is lying. If several people witness a crime, it is probative to look for similarities in their collective descriptions to substantiate the facts of an event but to keep in mind the contrasts between individual descriptions.