Frandsen, A. N. (1935). Mechanical ability of morons. Journal of Applied Psychology, 19(4), 371-378.
http://dx.doi.org/10.1037/h0061900
A group of 100 boys ranging in chronological age from 10.5 to 20 and in IQ from 50 to 75 at the Minnesota State School for feeble-minded were given sets I and II of the Minnesota mechanical assembly test. The average percentile on the mechanical assembly test for this group was 15.85, and 52% scored below the fifth percentile; however, the distribution overlaps the entire range of a normal population. (PsycINFO Database Record (c) 2016 APA, all rights reserved)