1. Skinner, B. F. Personal communication, October 21, 1974.
2. Wong, P. T. P., Finnie, L. M., & Grife, W. Secondorder operant under FR conditions. Paper presented at the annual meeting of the Psychonomic Society in Boston, Massachusetts, November 1974.
3. Wong, P. T. P. Higher order operant acquisition under FR conditions with extended training. Unpublished study, Trent University, 1975.