A-not-B error

A-not-B error (also known as "stage 4 error" or "perseverative error") is a phenomenon uncovered by the work of Jean Piaget in his theory of cognitive development of children. The A-not-B error is a particular error made by infants during substage 4 of their sensorimotor stage.

A typical A-not-B task goes like this: An experimenter hides an attractive toy under box "A" within the baby's reach. The baby searches for the toy, looks under box "A", and finds the toy. This activity is usually repeated several times (always with the researcher hiding the toy under box "A"). Then, in the critical trial, the experimenter moves the toy under box "B", also within easy reach of the baby. Babies of 10 months or younger typically make the perseveration error, meaning they look under box "A" even though they saw the researcher move the toy under box "B", and box "B" is just as easy to reach. This demonstrates a lack of, or incomplete, schema of object permanence. Children of 12 months or older typically do not make this error.