modified from BASIC code in:
Byers, J.A. 1996. Random selection algorithms for spatial and temporal
sampling. Computers in Biology and Medicine 26:41-52.
other related programs:
Byers, J.A. 1993. Randomization algorithms in BASIC for experimental
design. Computers in Biology and Medicine 23:167-176.
Byers, J.A. 1991. BASIC algorithms for random sampling and treatment
randomization. Computers in Biology and Medicine 21:69-77.