Our selected place for random sampling is too specific because we think that since we are students in Mae Fah Luang University, choosing the place where we normally go to is important, moreover, most students from the university comes to have their meals here as well. Furthermore, choosing three foods with the type of food as a comparison is also specific. The reasons are that (1) equipment and materials for the laboratory experiment is not enough for testing different kinds of food; and (2) we have limited time in doing the research.