Cases like Michaël Peterson’s in The Staircase highlighted the delicate nature of blood spatter analysis. After Peterson allegedly beat his wife to death on the famous staircase, a lengthy court trial tried to convict him based on blood spatter analysis. The evidence was inferential at best and the prosecution tried to claim that the various drops of blood point to cast off blood that was sprayed from the flailing of the murder weapon that was used to beat his wife to death. On the other hand, the defense argued that the blood drops found on the ceiling and wall of the stairwell were too inconsequential to suggest an entire line of cast off blood from only one small drop of blood.
Both arguments are reasonable; this is the problem with the accuracy of bloodstain spatter analysis. Because of the subjective nature of the process, blood spatter can be taken to illustrate many different situations at a hypothetical crime scene. The prosecutor or defendant could pick and choose little details that confirm their argument and only present those in court. It is at its core a subjective science that can be helpful for piecing together a crime scene but real analysis of blood should be left to presumptive and definitive blood tests to glean information.
Inversely, this something-from-nothing phenomenon of bloodstain spatter analysis is the beauty of it as a forensic discipline. Given only a small set of information, blood spatter analysis can give information about the handedness of the criminal, type of crime that involved the blood, number and chronological order of blows, positions during the crime and the time since death. Blood spatter analysis is a wealth of information when used correctly. However, these estimations are subjective at the bottom line. They are educated, albeit very well educated, guesses. As helpful as blood spatter analysis can be to solving cases, it is not scientific like other fields in forensics.
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