Some web sources attribute stanines to the U.S. Army Air Forces during World War II.
Psychometric legend has it that a 1–9 scale was used because of the compactness of recording the score as a single digit but Thorndike[1] claims that by reducing scores to just nine values, stanines "reduce the tendency to try to interpret small score differences (p. 131)".
The earliest known use of stanines was by the U.S. Army Air Forces in 1942.
[2] Test scores are scaled to stanine scores using the following algorithm: The underlying basis for obtaining stanines is that a normal distribution is divided into nine intervals, each of which has a width of 0.5 standard deviations excluding the first and last, which are just the remainder (the tails of the distribution).
The median lies at the centre of the fifth interval.