DEXA technology is considered the gold standard of Body Composition Technology. All other technology's accuracy compares to DEXA. The output provides a detailed results sheet.
For many years this was considered the only accurate way to get a body composition. It's high price-point ($150,000+) and a very large footprint makes it prohibitive for many businesses. It also does not measure an individuals water content, which is important for athletes.
BIA technology uses electricity to determine body composition. Any unit that is at least THREE frequency and EIGHT electrode is going to be accurate. Research quality units tend to be at least FIVE frequency.
This technology is dependent on the conduction of electricity. This means it is heavily dependent on body water. Body water is constantly changing, varying the results (but not the accuracy).
Every company has different algorithms. Algorithms are based on an average group of people and there is no such thing as an "average person". Any other the major manufacturers are going to have accurate algorithms. You won't go wrong with any unit that meets the three frequency, eight electrode model.
BIA technology varies greatly in price and can range from $3,000- $20,000. The level of detail in the results is the largest variable in the units.
For most businesses, BIA technology offers the combination of accuracy and affordability they are looking for.
Company's like Bod-Pod use air displacement to determine body composition. This technology has become outdated considering it is no more accurate than BIA technology and has a large footprint and high cost.
Water displacement relies on the subject getting under water and pushing air out of their lungs and maintaining it for a period of time so the proper measurements can be taken. It also requires a pool or adequate size body of water. Like air displacement, other methods, like BIA, has become more cost efficient without sacrificing accuracy.
New technologies are always coming to market making claims about measuring body composition. Most of these have not been clinically tested in a research environment by an independent organization.