Why do scientists only test one variable at a time?

Question

Why do scientists only test one variable at a time?

Answer 1

1. because it is easier.

2. For cross-referencing with other variables

Answer 2

If you test for more than one variable at a time, and there is a change, you won't know which variable is the cause of the change. By keeping all other variables constant and changing only one at a time, you can determine if that variable gives a change, and what the magnitude of that change is, without worrying about interferences.

Answer 3

Scientist PREFER to test only one variable at a time; that way any change can be linked to the one different variable.

In the real world, sometimes it isn't possible. Then one has to rely on statistics, regression analysis, which isn't as clear cut. And sometimes, for whatever reason, you have no control over variables, then just observation and statistics.