 Keller (2006) 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.