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Consideration of the ability of individual informants to accurately observe and report on behaviours associated with this disorder, is particularly important in studies using rating scales as the only method of assessment. As such, informants become the sole providers of information in the absence of diagnostic interviews or additional assessments by trained interviewers. Dependence is placed on the reliability and validity of rating scales to accurately measure the condition under study, and distinguish between normal and pathological samples according to the populations in which they are studied (either in clinical or community settings).

For the assessment and diagnosis of ADHD/HD, a number of issues are significant in this respect. Firstly, studies have shown that teachers and parents often give very different opinions of a young person's behaviour (eg Achenbach, McConaughy & Howell, 1987), which is likely to be attributable to the differing impact of the disorder that may occur in various situations (ie

classroom or at home). Secondly, drawing conclusions about patient groups identified using different rating scales based on differing diagnostic criteria (eg DSM-III or DSM-III-R) is complicated by changes in nosology (see Section 2.2.1.2), resulting in the identification of divergent groups with the disorder from similar populations (eg Newcorn, Halperin, Mealy, O’Brien et al., 1989). Thirdly, this disorder comprises three putative symptom domains (inattention, impulsivity and overactivity), as well as a number of associated areas of difficulty. These point to the necessity of using a multidimensional assessment tool, rather than relying on measures that have high levels of internal consistency in one domain only (Hinshaw, 1994). Fourthly, developmental considerations should be given to the age group of the population under study. Hinshaw (1994) recommends that due to the increased rates of the core symptoms of ADHD/HD during early school years, the thresholds should be raised to prevent over-diagnosis. He also recommends that due consideration should be given to the findings that severe levels of behaviour in this early age group are highly predictive of the continuity of disorder throughout the maturational process. Therefore, failing to include any of this group of young people, even if they report less severe levels of this disorder, would be an important oversight. Fifthly, as previously mentioned, parents and teachers may rate young people very differently according to gender (eg McGee & Feehan, 1991; Gaub & Carlson, 1997a), which may have implications regarding access to services. Finally, consideration should be given to levels of comorbidity which are found to be high in rating scales. For example, factor analysis has indicated that hyperactivity and conduct problems are strongly correlated (Hinshaw, 1987), as are inattention and hyperactivity (eg McGee, Williams & Silva, 1985). Whilst such studies may reflect the accurate interrelationships between inattention, hyperactivity and conduct problems, they may also be the consequence of items failing to adequately distinguish between them (Schachar, 1991).

As such, the methods used to identify cases for research purposes, particularly for ADHD/HD, can be problematic (Taylor et al., 1991). In

addition, although parents and teachers are generally perceived to be the best informants in the identification of hyperactive problems and inattention in childhood, either one can provide information on impairment and overactivity in more than one setting to fulfil ICD criteria, and some evidence of impairment in these settings to support DSM criteria. Inconsistencies in diagnosis are likely to occur, therefore, as a consequence of the different measures (considered above) as well as differences in the sources of information and the settings in which the behaviours occur.

Some of the inconsistencies in reports from parents and teachers involve information which may vary in degree and levels of severity, as well as sometimes completely contradicting each other. Yet these informants are vital in providing information about a child's behaviour, and are invariably the primary source of referral to Child and Adolescent Mental Health Services (CAMHS) during childhood. The usefulness of each informant has been disputed in a number of studies. For instance, some studies suggest that differences in reporting are attributable to the assessment tool used, whilst others have found that teacher information is more useful than parent information in distinguishing between ADHD subtypes, and between those who have and those who do not have the disorder (eg Loeber, Green & Lahey, 1990).

During adolescence the referral process begins to metamorphose and the primary source for referral and provision of information slowly begins to change. Increasingly, self-referral becomes part of the process and clinicians are likely to become more reliant on self-reported symptoms. This practice is likely to continue thereon into adulthood. Unfortunately, there is little evidence that adolescents are reliable informants of their own hyperactive-impulsive and/or inattentive behaviours. A study by Barkley, Anastopoulos, Guevremont & Fletcher (1991) showed that adolescents tended to underestimate their own behaviour, when compared to teacher and parent ratings, from which they concluded that adolescents were unreliable at self-report. Similarly, a later study by Schaughency, McGee,

Nada Raja, Feehan et al (1994) indicated that amongst adolescents aged 15 who did report symptoms of ADD, although these behaviours were likely to reach clinical significance and should not be dismissed, they were weakly correlated with parent reports of these symptoms. They concluded that parent and teacher reports continue to offer reliable information about symptoms even during mid to late adolescence. Support for these findings comes from a study by Danckaerts, Heptinstall, Chadwick & Taylor (1999), designed to validate a scale developed to identify hyperactivity in adolescents based on DSM-III-R (APA, 1987) criteria. As such, they found good inter-rater reliability and prediction of a number of outcomes (eg social adjustment, peer relationships, school functioning) from the scale. However, low levels of specificity and sensitivity from self-report, suggested that improved measures are needed and additional sources of information identified during adolescence and adulthood, in order to achieve reliable diagnoses of these types of behaviour. It has also been suggested that additional sources of information should be obtained, even when adults are referred for these behaviours.

These findings suggest that a multi-method approach to case ascertainment is adopted, combining information from parents, teachers and young people from which conclusions about diagnoses can then be drawn.