Sample - size needs for forestry herbicide trials ’

ZEDAKER, S.M., GREGOIRE, T.G., and MILLER, J.H. 1993. Sample-size needs for forestry herbicide trials. Can. J. For. Res. 23: 2153-2158. Forest herbicide experiments are increasingly being designed to evaluate smaller treatment differences when comparing existing effective treatments, tank mix ratios, surfactants, and new low-rate products. The ability to detect small differences in efficacy is dependent upon the relationship among sample size. type I and II error probabili t ies, and the coefficients of variation of the efficacy data The common sources of variation in efficacy measurements and design considerations for controll ing variation ate reviewed, while current shortcomings are clarif ied. A summary of selected tr ials estimates that coefficients of variation often range between 25 and lOO%, making the number of observations necessary to detect small differences very large, especially when the power of the test (1 l3) is considered. Very often the power of the test has been ignored when designing experiments because of the difficulty in calculating f3. An available program for microcomputers is introduced that allows researchers to examine relationships among sample size, effect size, and coefftcients of variation for specified designs, a and p. This program should aid investigators in planning studies that optimize experimental power to detect anticipated effect sizes within resource constraints.