When psychologists operationalise variables, they select aspects of behaviour that they wish to study and define them very carefully so they can be measured, either directly through observation or indirectly, through self-report, for example. Psychologists must be extremely careful that the way that they have operationalised their variables is appropriate, otherwise they may find that they are not actually measuring what they intended to measure. This would reduce the validity of their study. For example, Piaget operationalised the concept of object permanence by observing whether or not the infant actively searched for an object which was shown to them and then hidden. When children fail to search for a hidden object this may not be a sign that they do not have a mental representation of the object and therefore lack object permanence. It may simply be a sign that other visible stimuli become more interesting to them. They may believe that the object has been hidden for a reason, i.e. the adult wants them to concentrate on something else. When you are considering studies in developmental psychology, ask yourself is the researcher really measuring what they set out to measure or could the participant have behaved in this way for some other reason? If so, the study lacks validity.