Extraneous Variable

As we all know by now, psychologists like to control things -- in particular, we like to establish as much control as possible when conducting experiments. After all, what's the point of conducting the experiment if in the end we can't really say that the results are due to the variables we are studying? For example, if I want to study the effect of some new therapy to reduce blood pressure (an issue related to stress) wouldn't it be important to make sure that during the experiment I control as many other factors that are NOT part of the therapy so that in the end I can say that the results are due to the therapy and not, for example, me screaming at some participants during testing? Any factor or variable that causes an effect (or potential affects) other than the variable being studied is considered an extraneous variable.