The 4/17/13 usual time slot for CSI: Crime Scene Investigation instead aired the repeat "Elementary" episode titled 'The Leviathan'. Several times during the course of the show they stated 'if you eliminate the impossible what's left, no matter how improbable, is the truth'.
This mantra is exactly how many investigations are pursued only under the phrase 'scientific method' instead.
Basically, utilizing the scientific method, one reviews the facts and develops a theory or hypothesis. Then (especially in a scientific field like bloodstain pattern analysis which tends to be more subjective than an instrumental analysis for drugs or DNA or explosives) the scientist endeavors to disprove their theory.
It is a process of elimination really. Eliminating whatever isn't possible leaves only what is possible. I can't tell you how many times in court while testifying that I have been asked if some other explanation is a possibility. Having followed the scientific method and attempting to disprove the original theory, I was always able to testify that my experimentation established that several other suggested theories were not a possibility.
Obviously, this is not 100% of the time. Sometimes you may not be able to distinguish between patterns of expirated (blood being expelled from the body by force due to damage to or blood being present in the lungs or the airways) or impact (liquid blood being sent into flight by compression due to some forcible event) blood. But then you rely on the other facts of the case. Such as the injuries suffered by the victim. That may also help to eliminate some possibilities and narrow down what is 'probable'
