Researchers are studying the effects of two types of arthropods (thrips and mites) on the growth of a certain type of plant. Of particular interest is whether an infestation with both thrips and mites might result in less damage than infestations of either type alone since thrips may prefer to eat mites given the chance. They have planned an experiment with four treatment groups: thrips only, mites only, both thrips & mites, and no thrips & no mites. They will start a large number of plant seedlings and then will select 72 seedlings that are all healthy and approximately the same size. There are 24 cages available in a greenhouse. Cages are designed to grow plants, but will not let arthropods enter or leave the cage on their own. Three plants will be placed into each cage and then the cages will be placed onto 6 tables in a greenhouse with 4 cages per table. For each table, one cage will be randomly assigned thrips only, another assigned mites only, a third will be assigned both thrips and mites, and the fourth cage will be kept both thrip- and mite-free. After four weeks, each seedling will receive a score of 1 to 5 assessing its degree of damage where 1 means no damage and 5 means most severe damage including death. 1. 2. 3. 4. Specify the treatment factors & their levels, and then identify the treatment structure. [Be careful here - the true treatment structure may not be obvious from the description.] Identify the experimental unit and the observational unit. If there is more than one size of EU, identify the treatment factors corresponding to each size of EU. Identify the number of replicates per treatment group. Identify blocking factors (if any). If none, report there are no blocking factors.