FIREARM TOOL MARKS
Some of these are real. Fingerprints exist. They can be matched. Blood grouping (blood is "grouped" not "typed") can place an individual within a "race" or "gens" or "family" narrowing the range of suspects. Shoes and firearms do leave marks. But millions of people have the same blood "type" (O, AB, A, B; positive or negative), and even after identifying a dozen proteins to place the sample within a "race" or other group, many people have these proteins and they are inherited across groups. Moreover, we do not understand fully the mechanisms of mutation that cause these variations. Blood grouping is a statistical outcome only, not an identifier of an individual.
What makes these junk science is the way prosecutors use them in the courtroom, inflating their accuracy, precision, and reliability beyond reason.
Consider DNA evidence. The results are given as a percentage of likelihood. No such numbers are offered for fingerprints.
Meticulous cataloguing suggested that no two people had the same set of prints. That may well be true. We do not know for sure. This is an empirical truth only. It is the Problem of the Black Swan. Until the discovery of Australia, Europeans said that all swans are white. Like primitives for whom the sun has always come up in the east, but who have no knowledge of astronomy, we can take all the measurements we want, but we do not know the genomes and alleles for fingerprints; we know the what, but not the why.
Fingerprint examiners never disagree with each other in court. They only agree that the prints identify or are "inconclusive." In Suspect Identities, (Cambridge: Harvard University Press, 2001) Simon Cole tells the story (pp. 264-266; 272-274; 282) of experts in disagreement all of whose certifications were revoked because that was the only way for the International Association for Indentification to maintain the fiction that experts do not disagree about fingerprints.
The fact remains that fingerprints have been forged by several methods by police and by criminals. (The FBI first found a forgery by a law enforcement officer in 1925.)
These same problems are unanswered for matching bullets to guns, or tire treads, or shoe prints, etc., etc. No rigorous scientific study has established a statistically valid database.
Recovered memories and psychic evidence are even worse. Everyone knows that a new event or perception can trigger an old memory. When we meet our friends and relatives, we love to "remember the time when... " But we also know that memories change over time: they degrade and become confused. A skilled and forceful interrogator can plant memories in an accused suspect or an alledged victim. There are no scales or rulers or filters - there is no way to measure the results for accuracy. Belief in psychic phenomena is common across cultures, as are other superstitions. The CIA, KGB, and astronaut Edgar Mitchell all famously attempted to find scientific validity for psychic phenomena. None succeeded. Whatever our desire to believe, it is wrong to send people to prison based on folk tales and wishful thinking.
I say a bit more on my blog, Necessary Facts here .
LEARN HOW TO DO IT
When you practice these forensic techniques yourself, you learn the strengths and weaknesses of laboratory evidence. These are all basic skills for home, hobby, crafts and laboratory. Some of them - such as plaster casting - are centuries old and have many applications, aside from criminalistics.