An operational definition specifies concrete, replicable procedures designed to represent a construct. In the words of American psychologist Stanley Smith Stevens (1935), "An operation is the performance which we execute in order to make known a concept." For example, an operational definition of "fear" often includes measurable physiologic responses that occur in response to a perceived threat. Thus, "fear" might be operationally defined as specified changes in heart rate, electrodermal activity, pupil dilation, and blood pressure.