Which statement best distinguishes primary from secondary sources?

• A. Primary sources are always published in scientific journals.
• B. Primary sources interpret data from others.
• C. Secondary sources are always original experiments.
• D. Primary sources are original data or evidence, while secondary sources analyze or interpret them.

Answer: (D)

Primary sources present original data, observations, or artifacts collected directly by researchers. Secondary sources take those original materials and analyze, interpret, summarize, or synthesize them. So the statement that best distinguishes them is that primary sources are the original evidence, while secondary sources focus on interpreting or evaluating that evidence.

For example, a research article reporting new experimental results or a dataset you collected is a primary source. A review article, a textbook chapter, or a news story that discusses or summarizes those results is a secondary source. The distinction matters because you use primary sources when you want the original data or methods, and secondary sources when you want context, interpretation, or synthesis of multiple primary sources.

Other choices aren’t correct for this distinction: primary sources aren’t restricted to journals—they can be theses, conference papers, or datasets. Primary sources do not inherently interpret data from others; that role belongs to secondary sources. And secondary sources aren’t defined by conducting original experiments; they are defined by analyzing and interpreting other people’s work.