If two measurement variables are both found to be changing over time, what does this mean?

It means there is a causal link between the two variables.





It means there is a common cause of the changes in both variables (other than time).





It means that even though the two variables may be highly correlated, they could be completely unrelated in terms of cause and effect.





None of the above.|||It means none of the above. Just because two variables are changing does not mean causality or even correlation. For example, the temperature inside my house changes continuously and the stock market quotations also change continuously. They are not correlated or causal.|||Short answer: It could be any of the above.





Call your variables X and Y. If they are "changing in time" in a way that allows you to predict the value of X at some time T based on knowing the value of Y at some time T', you would say the two variables are correlated. (Other cases might require you to integrate or differentiate one or both of the variables with respect to T.)





This correlation could be because:


1.) X "causes" Y in some physical sense (like a change in electrical field causes a change to acceleration of charged particles).


2.) Y causes X (which can potentially be distinguished from (1) above if you can independently vary the value of X or Y and see what happens to the other).


3.) Some other phenomenon Z causes both X and Y (like gravitation causes both bending of light rays and force on nearby mass).


4.) X and Y are not causally related, but the perceived causality was coincidental or limited to some particular scope of time.