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Attention span is the amount of concentrated time on a task without becoming distracted. Most educators and psychologists agree that the ability to focus attention on a task is crucial for the achievement of one's goals.
Estimates for the length of human attention span are highly variable and depend on the precise definition of attention being used.
Attention span, as measured by sustained attention, or the time spent continuously on task, varies with age. Older children are capable of longer periods of attention than younger children.
For time-on-task measurements, the type of activity used in the test affects the results, as people are generally capable of a longer attention span when they are doing something that they find enjoyable or intrinsically motivating. Attention is also increased if the person is able to perform the task fluently, compared to a person who has difficulty performing the task, or to the same person when he or she is just learning the task. Fatigue, hunger, noise, and emotional stress reduce time on task. Common estimates for sustained attention to a freely chosen task range from about five minutes for a two-year-old child, to a maximum of around 20 minutes in older children and adults.
After losing attention from a topic, a person may restore it by taking a rest, doing a different kind of activity, changing mental focus, or deliberately choosing to re-focus on the first topic.
Many different tests for attention span have been used in different populations and in different times. Some tests measure short-term, focused attention abilities (which is typically normal in people with ADHD), and others provide information about how easily distracted the test-taker is (typically a significant problem in people with ADHD). 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, or as being inappropriate for some populations, or as 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. After the mother returned, the experimenter placed a new toy out of reach of the child, stating that the child could have it in a few minutes after the experimenter returned to the room. Both strategies were effective for delaying gratification, maintaining behavioral self-control and modulating feelings of distress," said Raver. 
Some authors, such as Neil Postman in his book, Amusing Ourselves to Death, believe that the attention span of humans is decreasing as modern technology, especially television, increases. Internet browsing may have a similar effect because it enables users to easily move from one page to another. Most internet users spend less than one minute on the average website. Movie reviewer Roger Ebert, an active blogger and "Tweeter," wrote of the effect of technology on his reading habits and his search for "frisson" on the web and in life. Ebert cited an article by Nicholas Carr in the June, 2010 Wired magazine about a UCLA professor, Gary Small, who used an MRI scan to observe the brain activity of six volunteers, three web veterans and three not. The professor found that veteran web users had developed "distinctive neural pathways."