To determine whether ADHD should be classified in three distinct DSM-IV diagnostic subtypes or a continuum of attention problems, maternal ratings of attention on the Child Behavior Check List (CBCL), in Durch boys at age 7, 10, and 12 years, were fitted to class models, assuming either subtype or severity differences. The fit of the models to the data is compared, to determine which model is appropriate. Researchers at the Universities of Notre Dame, IN; Vermont; Utrecht; and Amsterdam conducted the study. At all three ages tested, models that distinguish between 3 quantitative classes (mild, moderate, and severe attention problems) provide the best fit to the data. The attention problem (AP) severe class contains all the subjects diagnosed with ADHD-combined subtype. Some subjects with ADHD-predominantly inattentive type are in the moderate AP class. Factor mixture analyses show that the CBCL AP syndrome varies along a severity continuum of mild to moderate to severe attention problems. Children with ADHD are at the extreme of the continuum. Framers of DSM-V will need these data in considering a change in classification to a continuum rather than discrete diagnostic categories of ADHD. 
COMMENT. The proposed continuum of attention problems is not a novel concept for ADHD. Epstein MA, Shaywitz SE and associates  examined distinctions between ADD, LD, and ODD/CD. Children referred to mental health settings differ from those referred to child neurologists, and “may be considered an extreme of the continuum of ADD.” Many children with ADD will be represented by those referred primarily for ADD and LD, rather than those with ADHD and comorbid aggression referred for child psychiatry evaluation. Shaywitz BA and associates, defining and classifying learning disabilities and ADHD , report several lines of investigation showing reading ability and reading disability as a continuum. Awareness of this relationship of the norm to abnormal in a seamless relationship is critical to our understanding of the basis for reading disability (and ADHD). This concept might also provide evidence of a decreasing severity pattern with increasing age, and gender differences.
The present DSM criteria for diagnosis of ADHD rely on symptoms alone, and criteria dependent on signs (perceptual and neurological deficits, including EEG epileptiform discharges in 25% cases) are not admitted. Perhaps the new DSM-V diagnostic criteria dependent on grading of severity will include a reference to the neurobiological and genetic nature of ADHD and objective signs. A genetic overlap between measures of hyperactivity/inattention and mood is demonstrated in twins with comorbid ADHD and depression . Gene-environment interaction (genetic sensitivity to environmental factors) should also be considered in diagnosis and treatment.