This method utilizes a "Skin fold Caliper" to pinch the skin at several predetermined sites and measure the thickness of that pinch. The results are then summarized and evaluated via several published methods or equations. These methods are usually based on large population studies, which took skin fold and hydrostatic weighing measurements and then attempted to find equations that manipulate the skin fold measurements so that they match the hydrostatic weighing results. The reported validity of skin fold measurements is +/-6% at the very best and may be off much more due to several problems with measurement accuracy: Inter-operator error, measurements vary widely from technician to technician, technical errors, such as failure to identify the proper measurement site and failure to pinch only fat and no other tissue, the underlying assumption that subcutaneous fat represents the same percentage of total fat in every person, there are also problems opening skin fold calipers wide enough for very obese persons, resulting in underestimation.
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