Final answer:
Research studies, thesis, and dissertations are all considered as secondary sources in a literature review.
Step-by-step explanation:
Research studies, thesis, and dissertations are all considered as secondary sources in a literature review. According to the information provided, secondary sources rely on sources other than direct observation or firsthand experience, and they include books, articles online or in databases, and textbooks. These sources regularly cite and build upon primary sources to provide perspective and analysis.