Creating invariant subscales of the GHQ-30

Background: The GHQ-30 clearly contains more information than the single derived score. Attempts to tap this information using factor analyses of the scale items have largely been abandoned because the factors extracted depend on the population sampled. Method: It is first shown that factor analysis of the GHQ-30 for a given population at a given time is remarkably stable across subsets of the population, but not for the same population at different times. Different psychometric considerations are then invoked to define four subscales which are independent of the particular population and time of measurement. These subscales correspond to the four combinations of positive and negative responses to positively- and negatively-worded questions. Results: It is shown that these four subscales have very different characteristics in the population as a whole but that each has a stable distribution over time. Subscale profiles show qualitative differences between different age and sex groups and between different groups at high risk of psychiatric disorder. They also have different strengths of relationships with Neuroticism and with mortality. Conclusions: The four subscales provide more information than the single derived score. The GHQ-30 embodies a measure of positive mental well-being which is completely overlooked by conventional scoring and usage; this measure is worthy of further investigation. Our findings have implications for the development, use and interpretation of subscales derived from questionnaires, such as the GHQ-30, which measure changing states, rather than stable traits, in the individual.