Variation in infestation vitiates conclusions drawn from field tests unless the plat technique is able to compensate for such heterogeneity. The Latin Square arrangement of test plats proposed by Fisher has a number of advantages over the ordinary method of using strips across the area. The Latin Square is applicable to areas in which the variation from plat to plat occurs by approximately constant differences. It does not always compensate for heterogeneity when differences occur by chance or in approximately geometrical series. Careful studies of proposed test areas should be made to determine the type of variation present and no tests made except in those places in which a high degree of accuracy can be secured.