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I think you are on the right track to think about using a survey for this type of project, but you need to be careful to make sure that your survey is based on a scientific principle. However, I think this will make an excellent human behavior project. The independent variable will be your survey, the dependent variable are the responses to the survey. Your child will need a definite question and hypothesis before writing the survey.
First, I recommend doing a search using Google Scholar on "purchasing video games," and related topics to find scientific sources on this subject. Since these are primarily papers published in scientific journals, you will have to help your child understand as much as possible, but do try.
For example, here's an abstract of a paper on this topic.http://jar.sagepub.com/content/23/1/76.short
Here's an interesting reference that describes how video games are rated.http://www.it-c.dk/stud/projects_f2003/ ... apport.doc
You can look further to try to find articles that you have full access to. This should give you ideas for questions for your survey. Are you going to survey parents, young children, teenagers or video store employees? From your description of the problem, I think it would be interesting to survey parents and children to see if there is a difference in responses.
Here's information from the Science Buddies website on designing a survey science project:http://www.sciencebuddies.org/blog/2010 ... eysays.phphttp://www.sciencebuddies.org/science-f ... rvey.shtml
And, finally, since this is a project involving asking people questions, you need to review the rules for doing science fair projects with human subjects. You will need a signed consent form from each adult that you survey, and a signed form from the parents of any children who participate.http://www.sciencebuddies.org/science-f ... ects.shtml
Good luck. Please post again in this topic if you have other questions.