It is best to test one variable at a time. If you test two and a reaction occurs, it would be hard to know which variable made the reaction
It is still an equation.
Just one at a time
That one on the left.
It depends on the experiment. Normally only one is tested at a time because they can affect the experiment. Variables are tested in a controlled experiment to see whether they affect the outcome and also how.
Using just one drop of blood is a time-tested method used in blood-typing.
Controlled Experiment :]
One. If there is only one variable being tested, when you compare it to the control, you know that was the cause of a change.
Variables allow one to summarise a lot of information using just a variable and a related function.
operational varibles
Only the one being tested
There are many different things that are measured and tested by the data collected in an experiment. Color changes are one thing that can be measured and tested for example.
It would not be a fair test if there were 2 or 3