The serial subtraction of seven beginning with the number 100 is a conventional part of mental status evaluation of patients. My observations of its administration by medical students and residents clearly indicates they do not administer it as originally described, eg, not done as a timed test, nor do they know how to interpret the results obtained.
Hayman1 proposed the test in 1942 as a "two minute clinical test for measurement of intellectual impairment in psychiatric disorders," stating the following:
The test is administered orally, the subject being asked to take 7 away from 100, to take 7 away from the answer obtained, and so on. The subjects are placed as much at ease as possible and encouragement is freely given, but no assistance. Each subtraction is considered as a unit and calculations are made on the basis of the 14 possible correct subtractions, that is 93-86-79-72-65-58-51-44-37-30-23-16-9-2. Answers more
Manning RT. The Serial Sevens Test. Arch Intern Med. 1982;142(6):1192. doi:10.1001/archinte.1982.00340190148022