There is a science behind how to design a study. The first step is: constructing the right question.
Then, the next step is: what type of a study should it be? Should it be a case-control? Randomized Control Trial? Chart review?
Another key questions is: where do you find your subjects? How do you recruit?
How do kids act after playing Video games?
This study will show that video game play a role in how kids act after playing violant or non violant games.
Kids may think that after they have played video games that this is exceptable in the real world. They may feel as there will be no consequence if acting upon and will be able to start from the begining.
Every study will have biases, the goal should be to state and minimize the biases in each study.