Forensic analysis of most physical and biological evidence is conducted for two purposes: identification and comparison. Identification determines what exactly a particular item or substance is. Is that white powder cocaine? Is that brown stain dried blood? After testing, a forensic examiner may state that the substance in question is present, not present, or that testing was inclusive and the presence of the substance can't be ruled in or ruled out.
Comparisons are made to find out whether a known and a suspect item or substance share a common origin. Did the fingerprint, hair, or blood come from the suspect? Does the paint smudge found on a hit-and-run victim's clothing match that of the suspect's car?
After comparing a crime-scene fingerprint to one obtained from a suspect, the examiner may state that the two match, keeping the suspect in the hot seat, or that they don't match, thus exonerating the suspect as a source for the print. The examiner also may find the comparison inclusive, perhaps because the crime-scene print was of poor quality. When that happens, the suspect is neither cleared nor condemned.
Crime-scene investigators use evidence to create linkage, that is, a connection between a suspect and a person, place, or object. Finding a victim's hair on the clothing of the suspect, for example, suggests that they had some degree of contact and thus links the two together. A suspect's fingerprint, blood, or semen at the scene of a robbery, murder, or rape strongly links the suspect to the crime scene. A murder weapon on which a suspect's fingerprints are found requires a great deal of explaining. Each circumstance links elements (person, place, or object) of the crime to the suspect.
Investigators also attempt to link multiple crime scenes to a single perpetrator. For example, if a particular shoe impression or an identical fingerprint is found at two or more scenes, this evidence may link the scenes to a single suspect. Even if investigators don't have a suspect, such a linkage allows them to use other clues at the various scenes to create a better picture of the perpetrator they seek.
The skill of a crime-scene investigator in collecting evidence and the precision of the analytical procedures of the crime lab work in tandem to establish these links. When successful, evidence may find its way into court and result in a conviction.