P. T. Costa and R. R. McCrae use the NEO Personality Inventory to measure these factors: extraversion, anxiety, agreeableness, conscientiousness and openness. The identification of such factors, therefore, is necessarily only inferential and subjective, and powerful evidence of validity is required other than identification of factors by the items loading on them. Although there is some good evidence supporting two of the personality factors, extraversion and anxiety, this is still far less than that for the ability factors. Extraversion is conceived of as the behavioural, personality manifestation of the arousability of the central nervous system, specifically the ascending reticular system. These so called L factors formed the basis of his questionnaire factors, thus ensuring that these also sampled the whole population of personality variables. This work resulted in one of the best-known personality questionnaires, the Cattell 16 Personality Factor test. There are no problems with the first two factors; virtually all researchers regard these as two major personality questionnaire factors.