Great point. In the author's description he only discussed how to establish a correlation. However it's important to point out that in those types of studies you are not manipulating any variables in the equation which means it's very difficult to tease apart the different interactions in the system. If you really wanted to do DIY science you would need to change something and determine whether there was a change in the outcome with appropriate controls to make sure that your changes only produced an effect in the variable you wanted. This is the really fun part of science!!