The use of the Circadian Type Inventory as a measure of the circadian constructs of vigour and rigidity.

The Circadian Type Inventory (Folkard 1987) was administered to 191 students (150 females and 41 males). Corrected item to total correlations for both the Vigour and Rigidity scales were low to moderate. The internal reliabilities of the scales indicate that Vigour is statistically more homogeneous (0.74) than Rigidity (0.58). In the absence of the original CTI correlation matrix, replication using a 2-factor principal component varimax solution was undertaken. It explained only 26.7% of the variance. Post hoc factor analyses yielded 3 factors which explained 33.4% of the variance. The relatively low amounts of variance explained and the inadequate transfer of the circadian constructs of Rigidity and Vigour into question items does not recommend the use of the CTI to measure circadian rhythm characteristics in order to predict tolerance to shiftwork.