[1] The psychometric properties of the NPI have been continually investigated since its creation in 1979, both by original creators Raskin and Hall, as well as a variety of researchers to come, including: Emmons, Bushman & Baumeister, and Rhodewalt & Morf.
[1] The NPI has weak convergent validity and some items have been argued not to reflect the central dogma of narcissism (e.g. "I see myself as a good leader").
[1] On the other hand, research conducted by Raskin and Terry identified seven factors, also through PCA, including: authority, exhibitionism, superiority, entitlement, exploitativeness, self-sufficiency, and vanity.
It is primarily used in areas where high self-serving tendencies, positively correlated to narcissism, may be a very negative aspect in the job.
It has been shown that scores on the NPI are positively correlated with self-esteem, with some arguing that the test could be producing false-positives for healthy individuals.
[3] In addition to validity issues, the internal consistency of the NPI has been brought into question by del Rosario and White.