. 3 - A psychologist investigates the effect of instructions on the time required to solve a puzzle . Each of 20 volunteers is given the same puzzle to be solved as rapidly as possible . Subjects are randomly assigned , in equal numbers , to receive two different sets of instructions prior to the task . One group is told that the task is difficult ( X1 ) , and the other group is told that the task ( X2 ) . The score for each subject reflects the time in minutes required to solve puzzle . Test the null hypothesis at the 0.05 level of significance .