Understanding how to manipulate variables and control for potential threats to experimental validity can be the difference between a solid research study and a near meaningless study. Variables are the basis for all of the statistics you will perform on your data. If you choose your variables wisely and make sure to control for as many confounds and threats to experimental validity as possible, your study is much more likely to add to the knowledge base in your area of specialty. Assuring that the measurement devices used are both valid and reliable will also add a lot to significant results. When any of these is called into question, the entire study gets called into question. Once again, garbage in – garbage out.