Talk to your students clearly about academic honesty. Devote time in class and space in your course syllabus and assignments to define and discuss academic honesty and your expectations on class assignments. Make sure students understand what plagiarism is, and how easily it can be detected. Make sure they understand the role of good research habits and good time management in getting assignments done. The Center for Academic Integrity is a good starting place for ideas. See also the ISU Judicial Affairs website for ideas on incorporating academic honesty expectations into your syllabus.
Make sure students have the right skills and knowledge. Don't assume students already know how to do research or find authoritative information in your subject area, because chances are very likely they don't. Make sure students understand in advance how to find the type of research information you want them to use, and that they understand your expectations in constructing a references cited list. When students are not familiar with the research tools and databases you want them to use for their papers, or not familiar with distinguishing the types of sources (scholarly versus popular, etc) you want them to use, consult with your subject librarian to arrange an in-class session and / or a course-specific Instruction LibGuide.
Take action on academic misconduct. When you see it, deal with it and report misconduct to the Dean of Students Office every single time. The Judicial Affairs website has full information on the process to follow. Their website and others, such as those from the Center for Academic Integrity and Plagiarism.org, also have ideas on helping students understand definitions, consequences, and best practices for instructors and students alike to cultivate to avoid plagiarism.
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