" In the security guard industry, for example, electronic alarm systems represent a potent substitute. Moreover, they can only become more important since labor-intensive guard services face inevitable cost escalation, whereas electronic systems are highly likely to improve in performance and decline in costs. Here, the appropriate response of security guard firms is probably to offer packages of guards and electronic systems, based on a redefinition of the security guard as a skilled operator, rather than to try to outcompete electronic systems across the board. "

― Michael E. Porter , Competitive Strategy: Techniques for Analyzing Industries and Competitors