The Vice President for Human Relations at Widget Manufacturing Co

The Vice President for Human Relations at Widget Manufacturing Co. wanted to see whether a flex-time program would improve worker morale. He randomly assigned employees to two groups. He measured each group's job satisfaction; then he gave one group flexible working hours. The other group remained on fixed working hours. After six months, he again measured the degree of job satisfaction for both groups. During the six-month period, the Board of Directors granted all workers a six percent raise to quiet complaints. During his analysis, he found that all of the experimental-group subjects happened to be office employees and all of the control-group people were factory employees. In addition, a high turnover rate meant that only half of the factory workers who began the experiment were still with the firm six months later. He found that the experimental group had higher satisfaction. Which of the following threats to internal validity DID NOT occur during the experiment?

A) history effect
B) selection bias
C) mortality
D) statistical regression
E) all of the above