Subtle formal thought disorders are difficult to quantify. Their relationship to florid thought disorder is unknown.

To assess the interrater reliability, sensitivity and factor structure of a new assessment instrument, the Thought and Language Index (TLI), and to determine if minor aberrations detectable in the speech of healthy individuals are related to the more severe formal thought disorders characteristic of schizophrenia.

Interrater reliability was evaluated by determining the intraclass correlation for the ratings by five assessors. Factor analysis of the TLI scores of 87 patients was performed, and TLI scores in matched patients and controls were compared.

The intraclass correlation was good for individual TLI items, and excellent for sub-scale scores. Factor analysis identified three groups of approximately orthogonal disorders. Mild speech aberrations were observed in healthy participants and in patients with schizophrenia. The prevalence of mild aberrations was correlated with the prevalence of definite formal thought disorders.

The TLI is reliable and capable of detecting subtle disorders. Some mild aberrations occurring in the speech of healthy individuals appear to be attenuated forms of the florid disorders characteristic of schizophrenia.