Final answer:
Secondary sources are useful when conflicting primary authority exists, the issue is a closed-fact problem, or a researcher does not have access to primary sources.
Step-by-step explanation:
Secondary sources are useful when:
- Conflicting primary authority exists: Secondary sources can help to resolve conflicting information or interpretations found in primary sources.
- The issue is a closed-fact problem: Secondary sources can provide facts and information on a topic that is already well-established and does not require further research.
- A researcher does not have access to primary sources: Secondary sources can be used as a substitute when primary sources are not available or accessible.