Skin. A researcher is carrying out a study of the effectiveness of four different skin creams for the treatment of a certain skin disease. He has eighty subjects and plans to divide them into 4 treatment groups of twenty subjects each. The subjects are assessed and put in blocks of four, according to how severe their skin condition is; the four most severe cases are the first block, the next four most severe cases are the second block, and so on to the twentieth block.

The four members of each block are then randomly assigned, one to each of the four treatment groups. State whether the investigation in question is an observational study or a designed experiment. Justify your answer.