THE PLURAL FORMS TEST: A VISION-INDEPENDENT TEST OF PREMORBID INTELLECTUAL ABILITY

Tests of irregular word reading, NART (National Adult Reading Test) and TOPF (Test of Premorbid Intelligence), can reliably estimate premorbid IQ since word reading ability is both highly correlated with IQ and remains relatively intact in a diverse range of neurological conditions. However, such tests require intact vision, and should not be administered in standardised form in visually-impaired participants, necessitating the development of vision-independent tests. We developed the Plural Forms Test (PFT), a candidate set of 162 singular words assembled from linguistic corpora, which requires participants to state the plural form of orally delivered words. Healthy participants (N = 50) were recruited to evaluate PFT performance against objective measures of current IQ, the Wechsler Abbreviated Scale of Intelligence full-scale IQ (WASI-II FSIQ), and Raven’s Progressive Matrices (RPM), along with premorbid IQ tests, the NART, TOPF, and an oral presentation of Spot-the word (STW-2). Strong correlations with WASI-II (r = .82), RPM (r = .60), NART (r = .74), TOPF (r = .81), and STW-2 (r = .74) were found. Next, a genetic algorithm (GA) proposed in our earlier work was used to find a subset of PFT words to make the new test shorter and more clinically practical. The GA suggested a subset of 42 words, giving a PFT vs WASI-II FSIQ correlation of r = .95, with an MAE of 3.00 IQ points. Subject to validation in an independent sample, these findings suggests that the PFT is the most accurate psychometric proxy estimate of FSIQ currently available.