Effect of informant and instrument

Traditionally, information is obtained from parents and the child, and is then combined by the clinician on a case-by-case basis. However, the need for consistent diagnostic rules that are imposed by a 'menu-driven' approach can prove difficult, since the weight given to a particular informant may best vary according to condition. Thus, if a parent says a child has symptoms of conduct disorder but the child denies it, the parent is more likely to be right and the child may be covering up or ashamed. However, if the parent says the child is not depressed but the mental state examination of the child reveals otherwise, it is the parent who may be ignorant of their child's true state. Such difficulties reduce the validity of interviews which use invariant combination rules. Further, in genetic studies, the heritability of a condition may vary greatly according to which informant is believed. Thus, in the Virginia Twin Study, conduct disorder was 69 per cent heritable according to the information derived from the mother interview, 36 per cent using information from the child, and only 27 per cent using information from the father. (9) Studies such as these underline the need for clinically sensitive ways of combining information, and the use of multi-informant, multi-method ascertainment of information. Statistical techniques such as latent variable analysis may help reduce measurement error, but may build in unwarranted assumptions that distort the raw data.

Structured interviews which accept the respondent's reply do not require lengthy training or clinically informed investigators, and so are popular in epidemiological surveys. However, the quality of information differs little from that obtained by questionnaires, (i°) and often has a high false-positive and false-negative rate compared with semistructured interviews. Direct observation, although expensive, often provides the most reliable and valid information for assessment of disruptive disorders/.!1.'

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