The function of these materials is to interpret primary sources. They can be described as at least one step removed from the event or phenomenon under review. Secondary source materials interpret, assign value to, conjecture upon, and draw conclusions about the events reported in primary sources.
Remember, secondary information is being viewed and interpreted through the eyes of the writer/interpreter. This must be taken into account when one is attempting to critically assess the value of the source material.
The following questions might be asked to help ascertain the nature and value of secondary material: