Researchers at a university want to know if higher levels of nitrogen in fertilizer will increase the production of tomatoes per plant. twenty plants are given normal levels of nitrogen and twenty other plants are given ten percent higher levels throughout the growing season. the plants receive the same levels of sunlight, water and are planted in the same soil on one farm. at the end of the experiment the average number of tomatoes produced is the same for each group. the scientists repeat the experiment on two additional farms further south that season. the researchers conclude that increasing nitrogen levels by 10% in tomatoes is not beneficial.