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Chapter 3. Assessment of the reliability of the test battery

When testing reliability while a method is being developed, it is usual to study inter-rater and testretest reliability ( 148, 149). Estimation of inter-ruter reliability was considered relevant because the scoring of NPM-X requires some degree of subjectivity. Inter-rater reliability provides information on the method’s stability against variation in carrying out the test and use of scoring criteria between the testers. When the test procedures are strictly standardised, the error variance attributable to scoring differences is considered to be minimal. However, there is reason to expect more tester variance with tests when scoring necessitates an intensive individual examination and/or some degree of individual opinion (148). Estimation of tesr-retest reliability provides information on variation attributable to collection of information on two different occasions. This variation may be due to day-to-day variation of the child or to differences in testing conditions. If the intervals between tests are short and they are all carried out by the same person, any fluctuations are assumed to be chance and they are included in the error variance of the test scores. Learning between the first and second tests may also lead to differences in the test results, and the test should therefore be assessed specifically to see whether learning could lead to significant variance. Internal consistency, calculated as Cronbach’s alpha (which is the average of all possible split-half correlations), gives information about the content sampling, i.e. the homogeneity of the subscales in a test area or the test items in a test scale. A high alpha value indicates homogeneity and is of importance in order to achieve inter-rater and testretest reliability. Testing of consistency between the test items within each subscale has so far been postponed. A total of 69 main scales and subscales, scored on three or four dimensions, gave a total of 214 variables. It was therefore found practical to study the reliability of the main scales and subscales first. When the test battery is to be developed further, it might be relevant to study the internal consistency between items within each subscale (at any rate where there is low reliability), in order to see whether wrongly placed or poorly functioning items should be omitted.