An important aspect of collecting evidence is properly obtaining control samples, which are samples taken from a known source against which the examiner can compare samples taken from the crime scene.
Control samples may come from the victim, from the suspect, or from items found at the scene. A fiber found at the scene is most valuable when control fibers are available from the floor mats of the suspect's vehicle. That way, the known or control sample taken from the car can be compared with the unknown sample discovered at the crime scene.
A match either puts the suspect at the scene or the victim in the suspect's car. Control samples of blood taken from the victim and the suspect can be compared with blood from an unknown bloodstain found at the scene to determine whether either of them shed the blood.
Control samples sometimes are materials that are identical to those on which evidence was found. For example, a sample of charred carpet that is suspected of containing residue from a fluid used to start a fire is best compared against the same kind of carpet that is known to be free of the suspect materials. A carpet sample taken from an area left undamaged by the fire can often provide the known sample. If a suspicious chemical is present in the charred carpet but not in the known sample, the laboratory examiner can be more certain that it is indeed a foreign chemical and not a component of the carpet or its adhesive.