This test controls the type I comparisonwise error rate not the experimentwise error rate. The comparison-wise error rate is the probability of a Type I error in any comparison. According to the output, there is evidence of a difference between 15ppm and 35 ppm. Since the mean biomass at 30 ppm is not found to be different from that at 35 ppm, and that at 35 ppm is not found to be different from that at 50 ppm, there is no evidence of a difference in the mean biomass between 30 ppm and 50 ppm. The results of this experiment were interesting but not conclusive.
Every block is randomized separately from every other block.
Every treatment must appear at least once in every block. Blocks should contain experimental units that are as different as possible from each other. Department of Statistics, Simon Fraser University.