The document discusses different models of the relationship between anxiety and depression symptoms. It summarizes Clark and Watson's (1991) tripartite model, which proposes three clusters of symptoms - anxiety-specific, depression-specific, and a shared non-specific cluster. The thesis concludes the tripartite model is best supported by research. It examines the Mood and Anxiety Symptom Questionnaire (MASQ) but finds it does not validly assess the tripartite model. In response, the thesis proposes a new measure, the Tripartite Clinical Symptom Inventory (TCSI), which demonstrates reliability and validity in assessing the tripartite model factors through confirmatory analysis.

1. The relationship between anxiety and depression remains unclear, despite ongoing
revision of diagnostic concepts over the last sixty years. Various proposals have been
suggested which attempt to explain this relationship. Some have suggested that anxiety
and depression are distinct disorders, with different etiologies, symptoms, natural
courses, and response to treatment (the pluralist position). Others have suggested that
anxiety and depression are a single disorder, or at least different manifestations of the
same underlying pathology (the unitary position). Clark & Watson (1991) proposed an
alternative model in which anxiety and depression symptoms are divided into three
clusters — a specific set of symptoms for each, as well as a shared set of non-specific
symptoms seen in both disorders. They suggested that such a tripartite model of anxiety
and depression symptoms provided the best description of the manifestation of anxiety
and depression symptoms. Clark & Watson (1991) further suggested that by focusing on
the specific symptoms of anxiety and depression, the discriminant validity of
psychometric assessments of the two could be improved.
The current thesis reviews the literature in support of unitary, pluralist and tripartite
models of anxiety and depression symptoms, and concludes that a tripartite model is the
most consistently supported by the literature, including studies of symptoms, course,
treatment, etiology, family patterns and genetics in both. It examines in detail the
research into the tripartite structure of anxiety and depression symptoms, and concludes
that previous research also supports a three-factor structure of anxiety and depression
symptoms in existing self-report measures.
The Mood and Anxiety Symptom Questionnaire (MASQ) is reviewed as the only existing
clinical symptom measure based on the tripartite model. Although previous authors have
reported broad support for the MASQ, the present thesis uniquely used a sample of
anxious and depressed patients and confirmatory factor analytic methods, and failed to
support the MASQ as a valid tripartite measure. Specifically, there was no support at the
item or subscale level for any of the two-factor or three-factor models tested.
Furthermore, the MASQ subscales did not show the expected differences when
compared between anxious and depression patients, and the instrument was found to be
inadequate in discriminating between those diagnosed with anxiety versus those
diagnosed with depression.
In response to the psychometric inadequacy of the MASQ, the Tripartite Clinical
Symptom Inventory (TCSI) was proposed as a measure based on the tripartite model
which would be clinically useful, as well as showing strong reliability and validity. It was
also proposed that the TCSI would further validate several aspects of the tripartite theory
of anxiety and depression symptoms.
The original 45-item set was reduced over a series of exploratory factor analyses,
designed to yield a core subset of symptoms that were reliably and stably associated
with a single factor in a tripartite model. A total of 19 items comprised the depressionspecific (TCSI-D), anxiety-specific (TCSI-A) and general, non-specific (TCSI-G)
subscales. The three-factor structure of the TCSI^^ was affirmed in a heterogenous
sample, as well as showing factorial invariance across clinical and non-clinical samples,
using confirmatory factor analytic procedures.
The TCSI demonstrated strong reliability when assessed using measures of internal
consistency and temporal stability. In addition, the TCSI showed good validity, improving
on many aspects of the MASQ. The subscales of the TCSI retain sound convergent
validity, established through strong correlations with established measures of anxiety
and depression. More impressively, discriminant validity of the TCSI subscales was
shown to be good, especially when assessed using a clinical sample where item
variance was greatest. The clinical utility of the TCSI was also specifically investigated,

2. with the new instrument demonstrating excellent ability to differentiate between anxiety
and depressive disorders, as well as sensitivity to treatment effects.
The TCSI and the underlying tripartite model are highlighted as fruitful areas for future
research into the relationship between anxiety and depression. Furthermore, the ability
to identify specific and non-specific symptoms using the TCSI opens up avenues for
research into the specificity of different treatment for anxiety and depression to the
tripartite model symptom-clusters.
Keyword
Anxiety -- Classification.