  For persons with aphasia, naming tests are used to evaluate the severity of
the disease and observing progress toward recovery. The Philadelphia Naming
Test (PNT) is a leading naming test composed of 175 items. The items are common
nouns which are one to four syllables in length and with low, medium, and high
frequency. Since the target word is known to the administrator, the response
from the patient can be classified as correct or an error. If the patient
commits an error, the PNT provides procedures for classifying the type of error
in the response. Item response theory can be applied to PNT data to provide
estimates of item difficulty and subject naming ability. Walker et al. (2018)
developed a IRT multinomial processing tree (IRT-MPT) model to attempt to
understand the pathways through which the different errors are made by patients
when responding to an item. The MPT model expands on existing models by
considering items to be heterogeneous and estimating multiple latent parameters
for patients to more precisely determine at which step of word of production a
patient's ability has been affected. These latent parameters represent the
theoretical cognitive steps taken in responding to an item. Given the
complexity of the model proposed in Walker et al. (2018), here we investigate
the identifiability of the parameters included in the IRT-MPT model.

Methodology;Applications