The evaluation of psychological variable is the basic issue of management science. The usual assessment and scoring method for psychological variable is Likerttype scale. Not only the linear evaluation is subjective, but also it is inconsistent with the psychological law. Based on RT (reaction time) experiment, this paper discusses the difficulty of knowledge work as an example. According to the comparative analysis of both the RT data and semantic estimation, the evaluation and scoring of a psychological variable should be nonlinear, i.e. quantitative relation of the evaluation of psychological variable summit to Steven's law. The empirical research method and the power function evaluation of psychological variable model are useful efforts for objective evaluation of management variable.