A good measure of obesity is hard to find. Sophisticated measurements such as DEXA (dual energy X-ray absorptiometry), air displacement, and underwater weighing, are expensive and largely unavailable. Body mass index (BMI; the proportion of weight to height) is the most common measure of fatness, but it is inaccurate in weight-trained athletes. Waist circumference is difficult for most people to measure. Researchers from the University of Michigan found that neck circumference was related to excess body fat and was a reliable predicator of obesity in children. Neck circumference was a good screening tool in children for predicting future adult health problems, such as obstructive sleep apnea, diabetes and high blood pressure.