Journal Article: A comparison of frequency, interval, and time-sampling methods of data collection.

Repp, A.C., Roberts, D.M., Slack, D.J., Repp, C.F., & Berkler, M.S. (1976). Journal of Applied Behavior Analysis, 9, 501-508.

The link address is: http://dx.doi.org/10.1901/jaba.1976.9-501