A second model of incrementalist public policy is the group model. In these days of questionable campaign contributions and powerful vested interests, the notion of pressure groups and lobbies also has relevance. Another way of describing the group model is the hydraulic thesis, in which the policy is considered as a system of forces and pressures acting and reacting to one another in the formulation of public policy. An exemplary work that represents the group model is Arthur F. Bentley’s The Process of Government.15