The prevalence of attention-deficit/hyperactivity disorder (ADHD) has been estimated at 3-7% in the population. Children with this disorder are often characterized by symptoms of inattention and/or impulsivity and hyperactivity, which can significantly impact on many aspects of their behaviour and performance. This study investigated the characteristics of the "SWAN Rating Scale" and its discrimination of ADHD subtypes. This instrument was developed by Swanson and his colleagues and measures attentiveness and hyperactivity on a continuum, from attention problems to positive attention skills, using a seven-point scale of behaviour: "far below average" to "far above average". The Australian Twin Attention-Deficit/Hyperactivity Disorder Study consists of questionnaire data collected from families in 1990/2007. The Rasch model was used to measure the characteristics of items from the "SWAN Rating Scale"; how well these items discriminated between those with and without ADHD. The prevalence of each subtype was found to be 5.3% for inattentive ADHD, 4.3% for hyperactive ADHD and 4.6% for combined ADHD. A total of 14.2% of the cohort appeared to have ADHD. While the inattentive items appeared to be consistent with each other in their measurement behaviour and response patterns, the hyperactive items were less consistent. Further, the combined subtype appeared to be an entirely different type, with unique features unlike the other two subtypes. Further work is needed to distinguish the diagnostic features of each subtype of ADHD.