Tests like the DeGangi's "Test of Attention in Infants" (TAI) and Wechsler Intelligence Scale for Children-IV (WISC-IV) are commonly used to test for attention-related issues in young children when interviews and observations are inadequate. Older tests, like the Continuous Performance Test and the Porteus Maze Test, have been rejected by some experts. These tests are typically criticized as not actually measuring attention, being inappropriate for some populations, or not providing clinically useful information. Variability in test scores can be produced by small changes in the testing environment. For example, test-takers will usually remain on task for longer periods of time if the examiner is visibly present in the room than if the examiner is absent. In an early study of the influence of temperament on attention span, the mothers of 232 pairs of twins were interviewed periodically about the similarities and differences in behavior displayed by their twins during infancy and early childhood. The results showed that each of the behavioral variables (temper frequency, temper intensity, irritability, crying, and demanding attention) had a significant inverse relationship with attention span. In other words, the twin with longer attention span was better able to remain absorbed in a particular activity without distraction, and was also the less temperamental twin. One study of 2600 children found that early exposure to television (around age two) is associated with later attention problems such as inattention, impulsiveness, disorganization, and distractibility at age seven. This correlational study does not specify whether viewing television increases attention problems in children, or if children who are naturally prone to inattention are disproportionately attracted to the stimulation of television at young ages, or if there is some other factor, such as parenting skills, associated with this finding. How well a parent can capture and keep a two-year-old's attention on a toy may be more important than just a pleasant way to pass the time. "By successfully focusing a young child's attention on objects during free play, parents may be giving their child practice in using attention as a way to shift into a positive emotional state," said Raver. "We found that children whose parents actively directed and maintained their child's visual attention spent more time distracting themselves away from a source of distress." In one study, Raver observed 47 urban low-income mothers and their two-year-olds for ten minutes of free play, analyzing how much the pair kept each other's attention. The mother then left the room for four minutes and trained observers noted how the child managed his/her emotions.