of underwater weighing, but representing a densitometric method that is based on air displacement rather than on water immersion. Air-displacement plethysmography offers several advantages over established reference methods, including a quick, comfortable, automated, noninvasive, and safe measurement process, and accommodation of various subject types (e.g., children, obese, elderly, and disabled persons). However, its accuracy declines at the extremes of body fat percentages, tending to slightly understate the percent body fat in overweight and obese persons (by 1.68–2.94% depending on the method of calculation), and to overstate to a much larger degree the percent body fat in very lean subjects (by an average of 6.8%, with up to a 13% overstatement of the reported body percentage of one individual — i.e. 2% body fat by DXA but 15% by ADP). Near-infrared interactance A beam of infra-red light is transmitted into a biceps. The light is reflected from the underlying muscle and absorbed by the fat. The method is safe, noninvasive, rapid and easy to use. Dual energy X-ray absorptiometry Dual energy X-ray absorptiometry, or DXA (formerly DEXA), is a newer method for estimating body fat percentage, and determining body composition and bone mineral density. X-rays of two different energies are used to scan the body, one of which is absorbed more strongly by fat than the other. A computer can subtract one image from the other, and the difference indicates the amount of fat relative to other tissues at each point. A sum over the entire image enables calculation of the overall body composition. Expansions There are several more complicated procedures that more accurately determine body fat percentage. Some, referred to as multicompartment models, can include DXA measurement of bone, plus independent measures of body water (using the dilution principle with isotopically labeled water) and body volume (either by water displacement or air plethysmography). Various other components may be independently measured, such as total body potassium. In-vivo neutron activation can quantify all the elements of the body and use mathematical relations among the measured elements in the different components of the body (fat, water, protein, etc.) to develop simultaneous equations to estimate total body composition, including body fat. Body average density measurement Prior to the adoption of DXA, the most accurate method of estimating body fat percentage was to measure that person's average density (total mass divided by total volume) and apply a formula to convert that to body fat percentage.