In a golf club (say a driver), the shaft of the club is the "engine" that can make a difference in how a player hits the ball. Today, with the advent of Trackman, players can test equipment and have every detail of the ball flight recorded.

A golf club manufacturer is looking for the ideal set up in the shaft for a new driver they will be releasing that is aimed at the better player. The two aspects of the shaft that they are interested in are length and flex.

In theory, a longer driver shaft should generate more club head speed, but it can be more difficult to hit the ball in the center of the face. The flex of the shaft (how stiff it is) can dramatically change the spin rate of the ball, thus affecting distance.

They want to know the best combination to maximize distance. For the experiment, the golf company randomly chose 40 golfers from a country club's roster who had low golf handicaps.
a) 10 golfers with longer driver shafts and stiff flex
b) 10 golfers with longer driver shafts and regular flex
c) 10 golfers with shorter driver shafts and stiff flex
d) 10 golfers with shorter driver shafts and regular flex