The intelligence test is in some a crude instrument, for its assumptions can never be strictly met. The language environment of one home is never exactly that of another, the reading matter available to the subjects differs, and the stress upon cognitive goals varies.
Even the novel items depend upon perceptual criminations that may be acquired in one culture or subculture and not in another. Despite the difficulties, items can be chosen that work reasonably well. The items included in contemporary intelligence tests are those that have survived in practice test many others have been tried and found defective.