Suppose a researcher wants to study work productivity in a factory. Video cameras are installed to see how much time workers spend on task during a workday. The workers are more productive on the day...

Suppose a researcher wants to study work productivity in a factory. Video cameras are installed to see how much time workers spend on task during a workday. The workers are more productive on the day after the cameras are installed than on the day before. A possible cause of the increase in productivity that would represent a source of bias in the study is ____________________.