A mathematical index of performance on fixed-interval schedules of reinforcement.

@article{Fry1960AMI,
  title={A mathematical index of performance on fixed-interval schedules of reinforcement.},
  author={William Fry and R. T. Kelleher and Leonard Cook},
  journal={Journal of the experimental analysis of behavior},
  year={1960},
  volume={3},
  pages={193-9}
}