Terman adopted a convenient index of brightness that was suggested by the German psychologist William Stern (1871-1938). This index is the intelligence quotient, commonly known by its initials IQ. It expresses intelligence as a ratio of mental age to chronological age:
The 100 is used as a multiplier to remove the decimal point and to make the IQ have a value of 100 when MA equals CA. It is evident that if the MA lags behind the CA, the resulting IQ will be less than 100; if the MA is above the CA, the IQ will be above 100.