Around 8 years of age, patients usually are able perform a more difficult test called body plethysmography which is often called "a body box study". This nickname comes from the appearance of the equipment, which resembles a phone booth with a bench seat inside. The patient sits inside the box and breaths on a mouthpiece. Chest movements and changes in breathing pressures are measured, so the computer can calculate lung volumes. This test shows the total amount of air in the lungs and gives a more complete picture of a patients' breathing function. This test takes about 20 minutes and will be performed at least once a year during annual visits.