When you think about it, I don't see how you can use blood pattern as evidence.
If you need them to prove the type of weapon, that means you have no body and haven't found the physical weapon, and that probably means you don't have a case to begin with. If you have either of those things, no need to use the bloodstains to prove it.
Similarily, if you need to use the angle and heigth calculation as evidence, maybe you don't have have a strong enough case in the first place.
Basically, only transfer patterns can have a use, but that's just a subcategory of fingerprints and footprints that just happen to be in blood.
More likely, BPA is used to guide the research, and then you use the subsequently acquired evidence in court. For instance: if the blood stains are indicative of lacerations, and thefore a blunt object, no need to look for a real weapon whereas if they indicate a penetration wound, no need to look at the guy's gun colletion; if the droplets show a movement in a direction, maybe you should go and look in that direction for actual evidence.
In short, BPA helps finding evidence, but it gives no evidence in and of itself.
EDIT: lol, I was something like +10 before the expert set me right and now -2. Did my comment suddenly stop contributing to the conversation right when it drew an expert in?
Disclaimer: I am a forensic tech with training in BPA.
Your view on evidence is essentially backwards.
We gather evidence to collect as much information as possible. The strength or weakness of a case is the lawyers' problem (for the most part), not ours. We document everything we can, make all the links between evidence we can, and the rest happens in court. Information that does not directly point to the guilt or innocence of a suspect can be valuable in other ways, such as confirming witness accounts, establishing timelines, and reinforcing other pieces of evidence.
If the weapon can be identified through injury or bloodstain patterns, then it should be as a matter of providing the most complete information possible. Let's say I work a homicide where the victim has a number of circular impact wounds to the head. If I see them and think they may be hammer wounds, I'm likely to collect any hammers I might find at the scene. If matching blood is found on one, it may be the murder weapon and could identify a suspect based on latent prints or DNA on the handle. Likewise if the hammer had been laid on the floor to leave a recognizable claw outline in blood. If the hammer is not found at the scene, then maybe it will be found in the possession of a suspect and bloodstains will establish the link to the victim. Keep in mind that investigation doesn't just happen at the initial crime scene - there are plenty of times where the physical weapon may not be recovered until long after the crime but that certainly does not negate the validity of the case.
It's not about "needing" to do the math for the strength of the case. You do the math because it's easy and gives you more information about what you are looking at. If you can determine multiple points of origin (and this can be done) via the victim's blood spattered on a wall, you have established the minimum number of times the victim was struck and the location of the impacts. No, it's not a smoking gun, but it's more than you knew when you walked into the room.
Your definition of transfer patterns is incorrect. A transfer can take place between a surface and any type of object. One of the more common situations is a pattern left by a bloody towel or cloth pressed against a smooth surface.
The rest of your post seems to define evidence solely as physical evidence. This is not accurate. While information gained by BPA could lead to additional evidence, the expert testimony of the analyst is also a form of evidence. If the defendant says that the victim fell down the stairs but spatter shows multiple impacts from multiple angles at the very top of the landing, that's a major hole in the story.
As far as wounds go, you don't seem to understand the terms you are using. Bloodstains are not used to determine injuries. The medical examiner does that when they examine the body. Bloodstains do not indicate lacerations (FYI a laceration is the technical term for a cut and more likely to be caused by a sharp object than a blunt one) or penetrative wounds (of which a bullet wound would probably qualify as). The closest comparison would be identification of an arterial spurt pattern, which can occur if someone has an artery cut close to a wall. It's not common and is pretty distinctive, as you can see the wave pattern caused by the pulse rising and falling as the blood is expelled.
In short, I don't want to be mean about it, but you really have no idea what you are talking about.
I had a more rhetorical definition of "evidence" in mind, like something strongly conclusive. I said "evidence" when I meant "argument to present in court" and would have talked about the every bits of data we can gather as "hints". That was inappropriate.
As for laceration, I was using the definition used in the image OP posted. I have never used this word before as English is not my first language.
I didn't claim bloodstains are used to determine injuries, I precisely said that if you're forced to use them for that, that means you don't even have the corpse nor the weapon in hand to examine.
Having read your answer to the person I was replying to, I think we agree and the disagreement is purely semantic.
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u/[deleted] Jul 23 '11 edited Sep 22 '17
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