Both cohort effects (the birth year of the test-takers) and practice effects (test-takers taking the same form of IQ test more than once) must be controlled to gain accurate data.
Begge kohorte-effekter (fødselsår af test-tager) og praksis virkninger (test-tager tager den samme form for IQ-test mere end én gang) skal styres for at opnå præcise oplysninger.