A Manufacturer of Tennis Rackets Has Designed a New Racket

A manufacturer of tennis rackets has designed a new racket and would like to investigate whether players will prefer it to their previous design.The company obtains 100 volunteer tennis players.Volunteers are randomly assigned to one of two groups.Group 1 will use the company's previous racket first and the new racket second.Group 2 will use the new racket first and the company's previous racket second.Each player will serve 30 times with each racket.Then the percentage of good serves with the two rackets will be compared.The players are blinded to which racket they are using.Also,the person recording the percentage of good serves is blinded to which racket the players use.If the group of volunteers consists of 50 professional tennis players and 50 beginners,how could the experiment be improved?

A) Have players choose their group instead of assigning them randomly to a group.
B) Use blocking - first divide the volunteers into professional tennis players and beginners.Within each block randomly assign volunteers to one of the two groups.
C) Include a placebo.
D) Use matching - match each professional player with a beginning player before assigning players to one of the two groups.
E) Add another factor - level of player,so that there will be four treatments: new racket and beginner,new racket and professional,previous racket and beginner,previous racket and professional.