Intrinsic equivalence between developmental prosopagnosia questionnaires

The 20-item prosopagnosia index (PI20) is a self-report measure of face recognition ability, which is aimed to assess the risk for developmental prosopagnosia (DP), developed by Shah, Gaule, Sowden, Bird, and Cook (2015). Although they validated PI20 in several ways and it may serve as a quick and cost-effective measure for estimating DP risk (Livingston & Shah, in press; Shah et al., 2015), they did not formally evaluate its validity against a pre-existing alternative questionnaire (Kennerknecht et al., 2008) even though they criticized the weak relationship of the pre-existing questionnaire to actual behavioral face recognition performance. Thus, we administered the questionnaires to a large population (N = 855) and found a very strong correlation (r = 0.82 [95% confidence interval: 0.80, 0.84]), a principal component that accounted for more than 90% of the variance, and comparable reliability between the questionnaires. These results show unidimensionality and equivalence between the two questionnaires, or at least, a very strong common latent factor underlying them. The PI20 may not be greater than the pre-existing questionnaire; the two questionnaires measured essentially the same trait. The intrinsic equivalence between the questionnaires necessitates a revision of the view that the PI20 overcomes the weakness of the pre-existing questionnaire. Because both questionnaires contained unreliable items, we suggest, instead of using either questionnaire alone, that selection of a set of items with high reliability may offer a more robust approach to capture face recognition ability.