Final answer:
Comparing sources helps historians evaluate their reliability and credibility by identifying similarities and differences. It adds validity to information if multiple sources agree and prompts critical analysis if there are significant differences.
Step-by-step explanation:
Comparing sources is an essential step in a historian's evaluation of sources as it helps determine their reliability and credibility. By comparing multiple sources, historians can identify similarities and differences between them. If multiple sources agree on a particular event or detail, it adds validity to that information. On the other hand, if there are significant differences between sources, historians can investigate further to uncover potential biases or inaccuracies. For example, if one primary source describes an event in a completely different way than another primary source, it signals the need for critical analysis and evaluation of each source's perspective and reliability.