There are several models that attempt to categorize different types of followership. Kelly's model describes four types: sheep, who are passive and uncritical thinkers; yes people, who are active but also uncritical; survivors, who do just enough to get by; and effective followers, who are active, independent thinkers. Potter and Rosenbach's model places followers on axes of relationship initiative versus performance initiative, identifying politicians, partners, contributors, and subordinates. The Curphy-Roellig model evaluates followers based on critical thinking versus engagement, identifying criticizers, self-starters, brown-nosers, and slackers. Effective followership requires both independent, critical thinking as well as active participation. The nature of the leader