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Introduction
Should health anxiety be conceptualized as a dimensional (continuous) or a categorical (taxonic) construct? Although this question has been debated, to date no firm conclusions have been reached (see Warwick & Salkovskis, 1990; Hiller et al. 2002; Ferguson, 2005; Noyes, 2005; Marcus et al. 2007). Therefore, it has been argued that taxometric methods should be applied to address this question directly for health anxiety (Ferguson, 2005; Marcus et al. 2007). Taxometric methods represent a set of procedures designed to demonstrate the latent structure of a construct as either dimensional (i.e. distributed as a continuous variable, with individuals varying quantitatively from each other) or taxonic (i.e. individuals are differentiated into non-arbitrary groups or categories) (Meehl & Yonce, 1994; Meehl, 1995; Waller & Meehl, 1998; see Ruscio & Ruscio, 2004a for a review).
Dimensionality: implications for theory, research and practice
Ruscio et al. (2006) argue that whether or not a construct is dimensional or taxonic has important implications regarding the types of theoretical explanations offered for clinical and individual differences, as well as for the types of research methods and clinical practice used. Theoretically, a dimensional account implies the existence of additive multi-causal agents. By contrast, a categorical approach indicates the existence of either a single causal factor (e.g. single gene or threshold model) or multi-causal effects that produce a qualitative disjunction between taxons and compliments (e.g. emergenesis or developmental bifurcation). Clinically, a dimensional approach would mean a move towards empirically derived multiple divisions as a basis for differential diagnosis, rather than the use of arbitrary cut-offs (cf. Widiger & Trull, 2007). For research, a dimensional account implies using the full range of scores without splitting samples into 'cases' and 'non-cases' (cf. Marcus et al. 2004, 2006) and developing further dimensional psychometric assessments (Ruscio et al. 2006). Given these different implications, a direct test of the latent dimensional structure of health anxiety is required.
The dimensional debate in health anxiety
There is strong indirect evidence to support the position that health anxiety is dimensional. First, the same pattern of effects for health anxiety is observed on a variety of outcomes in both clinical and non-clinical samples (Marcus et al. 2007). This indicates that the theoretical models apply...