Controlling variables in an experiment affects the outcome and results. It is vital for a successful experiment that all factors are controlled except for the one being tested. If we stick with the cat example in section 1, we are trying to see if a stranger elicits more meows compared to an owner. So the one factor being changed is the person coming into contact with the cat. All other factors must be the same. These include the cat being tested, the room, the time, noise and light levels, how hungry the cat is likely to be, and even the clothes the owner and stranger wear. The more you think about factors which could affect results, the more you will find. Controlling the variables is the most crucial part of planning an experiment, so consider carefully.
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