The size depends on arousal level, time of day, room temperature, frequency of sexual activity, and differences of the measurement methods. In many studies penis length is measured along the upper side, from the base to the tip with the man standing and the penis held parallel to the floor. The circumference is measured on erect penis: just below the glans penis, in the middle of the shaft, and at the base. Self-measurements report a higher average length than those which used medical or scientific methods. In some studies measuring the length of stretched soft penis is used to estimate erect penis length. Soft penis length is not a reliable indication of erect length. 