Researchers are designing an experiment to compare two different types of running shoes, a and b, to investigate which type is better for minimizing running time for a one-mile run. The experiment will consist of distributing the shoes to runners who are classified as either professional or recreational. A randomized block design is planned, with blocking by classification of runner. Random samples of 50 professional runners and 50 recreational runners will be selected. Each runner within each classification will be randomly assigned to wear either the type a shoe or the type b shoe, and their running times will be recorded for a one-mile run.