Your class has decided to design an experiment to test the hypothesis that more insects will be trapped near the cafeteria than will be trapped near the gym or the library. five pit traps will be set at each location and left in place for three days. why must all conditions except the independent variable be held constant for all experimental treatments?

To avoid the effect of hidden variables to affect the outcome of your experiment and keep consistency among all of them.

Explanation:

For a scientific experiment to succeed you need to guarantee that there are no hidden variables that can affect the outcome of your experiment, this is why you what to set all of the five experiments as similar as possible, to reduce the effect of hidden variables and improve the likelihood of repeatability and future reproducibility.

The independent variable can not be held constant because it is the cause or reason for the outcome of your experiment; you can't predict how many flies are going to be in each pit trap, therefore, the independent variable changes.