Environmental health researchers are interested in the toxic
effects of copper and zinc. In one experiment, researchers 
randomly allocated 25 beakers of water containing minnow 
larvae to receive one of 25 treatment combinations (five 
levels of zinc for each of five levels of copper). After 
four days, the minnow were removed and analyzed for protein 
levels. The researchers suspect that higher levels of these 
metals will stunt the growth of the minnows, and subsequently 
lead to lower protein levels.