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CONTROLS

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p58-59

2026-06-20

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CONTROLS

Controls are materials whose source is known and which are used for comparison with unknown evidence. Controls are often used to determine if a chemical test is performing correctly. They may also be used to determine if a substrate where evidence may be found is interfering with a chemical or instrumental test. There are two types of controls: positive and negative. Consider a case in which some red stains are found on the shirt of a suspect in a homicide. The first question that needs to be answered about these stains is: Are they blood? A number of tests can be performed to determine whether a stain may be blood. Suppose one of these tests is run on some of the stains and the results are negative. There are a number of reasons why this might happen:

• The stain isn’t blood.

• The stain is blood, but the reagents used to run the test are of poor quality.

• Something in the shirt is interfering with the test. Before concluding that the stain isn’t blood, a number of additional steps could be taken. One might run a different presumptive test and see whether the results change. Another is to run the first test on a sample that is known to be blood and that should yield a positive test. This known blood is a positive control. It is a material that is expected to give a positive result with the test reagents and serves to show that the test is working properly. In this case, if the positive control yields a correct result, then it can be presumed that the reagents are working properly and there must be another reason for the negative result obtained on the blood-soaked shirt. It could be proposed that the shirt fibers contain some dye or other material that deactivates the blood test so that it will fail to react with blood. To test this hypothesis, some fibers from the shirt that have absolutely no stains on them could be collected and run the test on them. This would be a negative control for the shirt; it is expected that the results of the test would come out negative. If the test results are negative as expected, they could still mean that the shirt contains some thing that interferes with the test. This presumption could only be verified by running a different test on the stain. Other negative controls can be run on “blank” samples, that is, those prepared similarly to the test materials being used but without any sample present. If the initial test for blood was done on the stained shirt and came out positive, we should not immediately assume that the stain is definitely blood. A sample of the unstained shirt fibers should be tested as a negative control. A negative result here would mean that the positive result on the stain most likely means that the stain is blood. What is the consequence of not running a positive or negative control? If a negative control is not used, a false positive may be the result; that is, it may be concluded that the stain is blood when it is not. This gives rise to what statisticians call a Type I error. Type I errors are serious because they can cause a person to be falsely incriminated in a crime. Failure to run a positive control can cause a false negative result. This can give rise to what is called Type II error. This type of error means that a person may be falsely exonerated from a crime that he or she really did commit. Any error is problematic, but from the criminal justice standpoint, a Type II error is less serious than a Type I error. It is better to have someone falsely released than falsely accused. Positive and negative controls are usually easy to obtain and should be used to minimize the chance of errors.