Herzberg's two-factor theory has been critiqued. One criticism concerns the classification of the two factors. Data have not shown a clear dichotomization of incidents into hygiene and motivation factors. For example, employees almost equally classify pay as a hygiene factor and as a motivation factor. A second criticism is the absence of individual differences in the theory. Specifically, individual differences such as age, sex, social status, education, or occupational level may influence the classification of factors. A third criticism is that intrinsic job factors, such as the work flow process, may be more important in determining satisfaction or dissatisfaction on the job. Finally, almost all of the supporting data for the theory come from Herzberg and his students using his peculiar critical-incident technique. These criticisms challenge and qualify, yet do not invalidate, the theory. Independent research found his theory valid in a government research and development environment. Herzberg's two-factor theory has important implications for the design of work, as discussed in chapter 14.