Control Sample
Definition
A water sample collected from the suspected drowning environment at or shortly after the time of body recovery. Its diatom assemblage is the reference against which the organ samples are compared: a meaningful positive result finds the same taxa in both.
- Primary purpose
- Rule out substrate contamination and establish that questioned-scene matches are distinctive, not trivially replicated by background material
- Collection principle
- Samples taken from known, uncontaminated areas near evidence or from legitimate access locations, not from the suspected source
- Evaluation method
- Questioned sample compared against both control and crime scene sample to confirm the match is forensically meaningful
Common questions
What is a control sample?+
A reference sample collected from a known location or uncontaminated substrate. Controls are compared against questioned samples to establish whether matches are meaningful or just typical background variation found everywhere.
Why do forensic investigators collect control samples?+
Controls rule out contamination from the substrate itself and demonstrate that a positive match between a questioned sample and the crime scene is distinctive. Without controls, a match could just be coincidence if the same material appears in many places.
Where do investigators collect control samples from?+
From uncontaminated areas near the questioned evidence (like unstained cloth next to a bloodstain or undisturbed soil from the scene) or from locations the suspect legitimately accessed. The location depends on what the investigator is trying to rule out.
Related terms
- Chain of Custody
- The documented chronological record of who collected, handled, transferred, and examined a piece of evidence. For digital evidence, chain of custody includes...
- Acid Digestion
- A laboratory preparation step in which concentrated mineral acids are used to destroy all organic tissue in a sample, leaving only acid-resistant...
- Ante-Mortem Drowning
- Drowning that occurs while the victim is still alive and the cardiopulmonary system is functioning. Active breathing and circulation are prerequisites for...
- Bayesian Inference
- A framework for updating probability estimates in the light of new evidence; in forensic science, used to combine the prior probability of...
- Bone Marrow
- The fatty and haematopoietic tissue inside long bones such as the femur and tibia. Because it is enclosed in dense cortical bone,...
- Class Characteristics
- Features deliberately or systematically imparted to a group of items by design or manufacture, such as the calibre of a bullet, the...
- Conditional Evidence
- Evidence whose meaning comes from the state or condition of a scene at the time of the crime, not from the physical...
- Cross-Section Sample
- A soil sample collected to preserve the depth stratification of the profile, with distinct layers kept separate. Comparison of a questioned sample's...
- ENFSI ENG-FG1
- The European Network of Forensic Science Institutes' guidelines for forensic geological evidence, published in 2018 by the Expert Working Group on Forensic...
- FBI Soil Examination Unit
- The specialist unit at the FBI Laboratory in Quantico, Virginia, that has conducted forensic soil comparisons in federal criminal cases since the...
- Form 21 (Viscera)
- Standard Indian forwarding form for viscera and biological samples sent to the FSL toxicology division, listing exhibits, preservatives, FIR number, autopsy surgeon...
- Hematogenous Dissemination
- Spread of a substance via the bloodstream. In the diatom test, diatoms aspirated into the lungs during ante-mortem drowning enter pulmonary capillaries...
Explained in these topics
- Physical Evidence: Nature, Types, Collection and Chain of CustodyA known reference sample collected from a substrate near the questioned stain (unstained cloth, soil from an undisturbed area) to rule out contamination from t...
- Soil and Sediment as Trace EvidenceA reference sample collected from locations a suspect had legitimate access to or could plausibly have visited, used to evaluate how distinctive the match betw...
- Soil Comparison Casework and Statistical InferenceA reference soil collected from a location that is not the proposed source, used to characterise the local background variability and to demonstrate that the q...
- The Diatom Test in Drowning InvestigationA water sample collected from the suspected drowning environment at or shortly after the time of body recovery. Its diatom assemblage is the reference against...