As any middle school teacher can testify, anthropometric measures such as height and weight, and visible signs of sexual maturity vary widely from one adolescent to another, so that general clinical examination has limited value for age estimation. The skeletal examination is considered more reliable. In the reference radiological method, X-ray examination of the left hand and wrist is compared with standard images from an atlas published by Greulich and Pyle, who collected standards obtained in the 1930's and 1940's from an American white middle-class population. The main criteria applied for dental age determination are the eruption and development of the third molars, based on external and X-ray examination.