and Mackenzie, the predictor variables, as well as the job attitude and role perception variables, were both taken from individual employees, while the performance measures were taken from supervisors. They found that their predictors accounted for a higher proportion of variance in job attitudes and role perceptions than in employee performance. They posit the reason for this was that their predictors shared a common source with the job attitudes and role perception criterion measures, but not with the employee performance criterion measures. They suggested that common-source bias may not be the only reason for this, but that it should be controlled in future research. Lack of longitudinal studies Keller conducted a longitudinal study in which he pointed out that the vast majority of studies conducted on substitutes for leadership theory are cross-sectional in nature, making it more difficult to discern a causal relationship between substitutes and their effects on employee performance. Cross-sectional research is focused on finding relationships between variables at a specific point in time, whereas longitudinal studies involve taking multiple measures over a longer period of time. Keller's longitudinal study of the effect of substitutes on Research & Development teams found that two of the seven substitutes (ability and intrinsic satisfaction) affected team performance over time. Conceptual weakness Yukl (1998) pointed out that it is hard to identify specific substitutes and neutralizers for broad behavior categories, and that an improvement on the theory would be to use more specific behaviors in place of "supportive and instrumental leadership behaviors." Yukl said that recent studies testing the theory have used specific behaviors (e.g., contingent reward behavior and role clarification), however, the development of the theory does not reflect these developments. Closely related constructs Contingency model The contingency model of leadership was developed by Fred Fiedler in the 1960s after Fiedler spent 12 years collecting data from over 800 groups of employees. This model predicts that the most effective method of leadership will vary depending on the situation. A study that supported the model was one Fiedler did with 48 petty officers and 240 recruiters at a Belgium naval training center (Fiedler, 1965). Fiedler found that a leader's fit with the group and the task was more important in predicting outcomes than the leader's characteristics. For example, controlling leaders went best with heterogeneous groups with low position power. (Fiedler, 1965). There are many different theories within the contingency paradigm, which differ