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International System of Units
Forensic scientists, like all scientists, report numeric measurements using the International System of Units, more commonly known as "SI." Forensic scientists commonly use SI measures of meters for length, kilograms for weight and moles for amounts of a substance. Although the SI system prefers Kelvin for temperature measurements, these standards also list degrees Celsius as acceptable, and this is the numeric temperature measure used in forensics. Compound measures rely on a combination of these units--for example, density is expressed as kilograms per cubic meter. The standard metric prefixes also apply, and forensic scientists often record numeric measurements using smaller units such as centimeters or milligrams.
Measuring Human Remains
Numeric measurements of human remains use standard SI units but are usually recorded in centimeters. When only a portion of the corpse or only a skeleton is present, these numeric measurements help forensic scientists to determine the height, sex and age of the victim. Corpse measurements usually include total height; distance from ankle to hip and knee to ankle; foot, thigh, arm, forearm, upper arm, and index finger length; and wrist, neck, and leg circumference. When only a skeleton is present, these measurements are limited to the length of the radius bone in the forearm, humerus in the upper arm, tibia in the lower leg and femur in the upper leg. Forensic scientists use a formula of the standard ratio between the lengths of these long bones to extrapolate the victim's total height.
Measuring Bullet Caliber
Numeric measurements of bullets provide one exception to the use of SI measurements as a standard in forensic science. Scientists often measure bullets in both millimeters and inches, because bullets and guns are sold by caliber, which is measured in inches in the U.S. For example, a 22-caliber bullet measures 0.22 inches in diameter. However, weapons not made in the U.S. are made to metric standards, such as the 9-millimeter handgun, which uses bullets that measure 9 millimeters in outside diameter.
Standards and Calibration
In addition to direct measurements, forensic scientists also obtain many numeric measurements from lab equipment--for example, a mass spectrometer is used to determine the chemical composition of trace evidence. Forensic experts quantify these measurements relative to a known standard substance in order to reduce error--the standards are available from reliable laboratory supply companies. Forensic labs also hire experts to regularly calibrate their machinery in order to ensure that the numeric measurements produced by the machine are highly accurate and repeatable. Forensic science requires careful attention to standards and calibration, because lawyers may challenge the accuracy of numeric measurements in court.
Error and Uncertainty
Forensic scientists must report numeric measurements along with an estimate of their margin of error or uncertainty. This is especially important when reporting numbers obtained from lab equipment, which has an inherent margin of error. As a result, a measure of volume might be given as 20 milligrams +/- 0.0001 milligrams, meaning that the measuring device (e.g., balance) was calibrated to be accurate to within plus or minus 0.0001 milligrams.