The principle characteristics of ADHD are inattentiveness, hyperactivity and impulsivity. These symptoms tend to appear over several months, often with the symptoms of impulsiveness and hyperactivity preceding those of inattention, which may not emerge for a year or more. Different symptoms may also appear in different settings depending on the demands that the situation may pose for the child's self control. A child who "can't sit still" or isdisruptive will be noticeable in school, but the inattentive daydreamer may be overlooked. The impulsive child, who acts before thinking, may be considered a "discipline problem," while the inattentive child, who is sluggish and passive, may merely be viewed as unmotivated or "lazy." Since these symptoms vary within the population and across different settings, ADHD is not easy to diagnose.