Demographic characteristics and mean BNT data are presented in Table . Participants ranged in age from 22 to 92 years, and the majority were Caucasian (>95%). BNT scores were significantly correlated with age (r = −.21, p < .001) and years of education (r = .28, p < .001), but not with sex (r = −.10, p = .10). As expected, internal consistency was high (alpha = 0.91). Exploratory factor analysis revealed a 5.3:1 ratio between the first and second eigenvalues. A single-factor CFA using ADF estimators returned non-admissible solutions, but the use of robust maximum-likelihood estimation yielded a well-fitting single-factor model (CFI = 0.97; RMSEA = 0.0229; Satorra–Bentler scaled χ2 = 1717.61, p < .001). A two-factor model did not result in improved fit. Moreover, the result from DIMTEST (T-statistic = 0.99, p = .16) was consistent with the prior dimensionality assessments. Altogether, these findings suggest that the BNT was sufficiently unidimensional to proceed with IRT modeling.
Demographic characteristics and BNT data for 300 patients
BNT total scores ranged from 22 to 60. As shown in Fig. , participants provided 100% correct responses to four items (BNT item numbers denoted in parenthesis): “bed” (1), “tree” (2), “toothbrush” (10), and “hanger” (15). “Protractor” (59) had the fewest correct responses (19%). The graph in Fig. also illustrates multiple dips or points at which there is a prominent decline in the percent of correct responses for consecutive items. For example, 92% of participants responded correctly to “wreath” (28) and 88% responded correctly to “harmonica” (30), but only 63% responded correctly to “beaver” (29). Similarly, 81% responded correctly to “asparagus” (49), yet 41% responded correctly to the following item, “compass” (50).
Mean percent correct item responses on the BNT.
Table presents the IRT item discrimination and difficulty parameters. As expected, there was no variance associated with the four items with 100% correct responses. The standard error for items with highly skewed response patterns (e.g., “scissors”, “broom”) could not be defined under maximum-likelihood estimation. Protractor (59) had a negative, near-zero discrimination parameter, suggesting that it was a poor item yielding minimal-to-no psychometric information for the IRT model.
Item discrimination and difficulty parameters for the BNT
Among the remaining items, “comb” (7) showed the highest magnitude of discrimination, followed by “racquet” (21), “saw” (9), “canoe” (26), and “wheelchair” (16). The least discriminating items were “flower” (8), scissors (6), “latch” (51), “yoke” (56), and “trellis” (57). These findings are more clearly visualized in Fig. , where the items with the highest degree of discrimination show the steepest slopes, and those with the lowest discrimination have relatively flat slopes.
Matrix of ICCs for the BNT (Note: ICCs not available for items 1, 2, 10, and 15).
In terms of difficulty, “abacus” (60) exhibited the highest parameter and was followed by compass (50), yoke (56), palette (58), and sphinx (55). As noted earlier, although 81% of participants responded incorrectly to protractor (59), its IRT parametric difficulty could not be properly estimated because the likelihood of responding correctly was nearly equal for any individual along the ability spectrum. Besides the four items in which all participants responded correctly, the next five easiest items were flower (8), scissors (6), broom (12), camel (17), and house (4). Several items had difficulty parameters that suggested a notable discrepancy from their ordered placement on the test. For instance, “acorn” (32) was the 19th easiest item and harp (38) the 22nd easiest item. In contrast, “octopus” (13) was the 36th easiest item and seahorse (24) the 48th easiest item. These results highlight the lack of monotonic increase in psychometric difficulty among successive items.
Figure displays the global test information curve. The BNT provided the most information (reliability) for individuals in the low-average range of naming ability, or approximately −1.0 standardized units. Measurement error increased considerably when assessing individuals with at least a high-average degree of naming ability.
Test information and standard error curves for the BNT.