Trait variables are viewed as stable personality characteristics, and
state variables denote behaviour in specific situations.
See Cox and Wermuth (1996).
References
Cox, D. R. and Wermuth, N. (1996). Multivariate
dependencies. London: Chapman and Hall.
Cox, D.R. and Wermuth, N. (1990). An approximation to maximum likelihood
estimates in reduced models. 77(4), 747-761.